Filed by: ccoroiu [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=283337734&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/IR0DgpdJT_C3StAK4caJDw/runs/0/artifacts/public/logs/live_backing.log
Reftest URL: https://hg.mozilla.org/mozilla-central/raw-file/tip/layout/tools/reftest/reftest-analyzer.xhtml#logurl=https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/IR0DgpdJT_C3StAK4caJDw/runs/0/artifacts/public/logs/live_backing.log&only_show_unexpected=1

[task 2020-01-03T13:04:33.541Z] 13:04:33 INFO - TEST-START | /custom-elements/upgrading.html
[task 2020-01-03T13:04:33.541Z] 13:04:33 INFO - Run 0/10
[task 2020-01-03T13:04:33.576Z] 13:04:33 INFO - Setting pref dom.webcomponents.elementInternals.enabled (true)
[task 2020-01-03T13:04:33.783Z] 13:04:33 INFO - PID 5040 | ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to c:\users\task_1578054107\appdata\local\temp\tmpokdbo7.mozrunner\runtests_leaks_5240_tab_pid1500.log
[task 2020-01-03T13:04:33.810Z] 13:04:33 INFO - PID 5040 | [Child 1500, Main Thread] WARNING: No CID found when attempting to map contract ID: file z:/build/build/src/xpcom/components/nsComponentManager.cpp, line 721
[task 2020-01-03T13:04:33.837Z] 13:04:33 INFO - PID 5040 | [Parent 1428, Main Thread] WARNING: we only accept nsIURI interface type, patch welcome: file z:/build/build/src/dom/ipc/PropertyBagUtils.cpp, line 112
[task 2020-01-03T13:04:33.837Z] 13:04:33 INFO - PID 5040 | [Child 2528, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannelChild, this is likely broken: file z:/build/build/src/netwerk/ipc/DocumentChannelChild.cpp, line 63
[task 2020-01-03T13:04:34.012Z] 13:04:34 INFO - PID 5040 | JavaScript error: http://web-platform.test:8000/custom-elements/upgrading.html, line 201: NotSupportedError: Custom element upgrade to 'shadow-disabled' is disabled due to shadow root already exists
[task 2020-01-03T13:04:34.027Z] 13:04:34 INFO - PID 5040 | JavaScript error: http://web-platform.test:8000/custom-elements/upgrading.html, line 217: too much recursion
[task 2020-01-03T13:04:34.027Z] 13:04:34 INFO - PID 5040 | JavaScript error: http://web-platform.test:8000/custom-elements/upgrading.html, line 217: too much recursion
[task 2020-01-03T13:04:34.027Z] 13:04:34 INFO - PID 5040 | JavaScript error: http://web-platform.test:8000/custom-elements/upgrading.html, line 217: too much recursion
[task 2020-01-03T13:04:34.027Z] 13:04:34 INFO - PID 5040 | JavaScript error: http://web-platform.test:8000/custom-elements/upgrading.html, line 218: too much recursion
[task 2020-01-03T13:04:34.027Z] 13:04:34 INFO - PID 5040 | Assertion failure: !mCustomElementDefinition, at z:/build/build/src/dom/base/CustomElementRegistry.cpp:171
[task 2020-01-03T13:04:37.034Z] 13:04:37 INFO - NoSuchWindowException on command, setting status to CRASH
[task 2020-01-03T13:04:37.062Z] 13:04:37 INFO - mozcrash Copy/paste: Z:\task_1578054107\build\win32-minidump_stackwalk.exe c:\users\task_1578054107\appdata\local\temp\tmpokdbo7.mozrunner\minidumps\da21ba89-725e-480d-9ef6-e2fa31a49827.dmp Z:\task_1578054107\build\symbols
[task 2020-01-03T13:05:01.240Z] 13:05:01 INFO - PID 5040 | #01: static mozilla::dom::CustomElementRegistry::Upgrade(mozilla::dom::Element*, mozilla::dom::CustomElementDefinition*, mozilla::ErrorResult&) [dom/base/CustomElementRegistry.cpp:1193]
[task 2020-01-03T13:05:01.240Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.241Z] 13:05:01 INFO - PID 5040 | #02: mozilla::dom::CustomElementUpgradeReaction::Invoke(mozilla::dom::Element*, mozilla::ErrorResult&) [dom/base/CustomElementRegistry.cpp:54]
[task 2020-01-03T13:05:01.241Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.241Z] 13:05:01 INFO - PID 5040 | #03: mozilla::dom::CustomElementReactionsStack::InvokeReactions(AutoTArray<RefPtr<mozilla::dom::Element>,3>, nsIGlobalObject) [dom/base/CustomElementRegistry.cpp:1389]
[task 2020-01-03T13:05:01.241Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.242Z] 13:05:01 INFO - PID 5040 | #04: mozilla::dom::CustomElementReactionsStack::PopAndInvokeElementQueue() [dom/base/CustomElementRegistry.cpp:1280]
[task 2020-01-03T13:05:01.242Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.243Z] 13:05:01 INFO - PID 5040 | #05: mozilla::dom::CustomElementReactionsStack::LeaveCEReactions(JSContext*, bool) [dom/base/CustomElementRegistry.h:285]
[task 2020-01-03T13:05:01.243Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.243Z] 13:05:01 INFO - PID 5040 | #06: mozilla::dom::CustomElementRegistry_Binding::upgrade(JSContext*, JS::Handle<JSObject >, void, JSJitMethodCallArgs const&) [s3:gecko-generated-sources:6582e922cc8325cb3bf86bbcd814111ec22a422ae2c18bd64c0e1d5d2bed7a2854193057df9a54e2df6d05aa276e8f74e2211ee173eef27287745616595c52d3/dom/bindings/CustomElementRegistryBinding.cpp::474]
[task 2020-01-03T13:05:01.243Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.244Z] 13:05:01 INFO - PID 5040 | #07: mozilla::dom::binding_detail::GenericMethod<mozilla::dom::binding_detail::NormalThisPolicy,mozilla::dom::binding_detail::ThrowExceptions>(JSContext*, unsigned int, JS::Value*) [dom/bindings/BindingUtils.cpp:3151]
[task 2020-01-03T13:05:01.244Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.244Z] 13:05:01 INFO - PID 5040 | #08: ??? (???:???)
[task 2020-01-03T13:05:01.244Z] 13:05:01 INFO - PID 5040 | #09: ??? (???:???)
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO - PID 5040 | #10: ??? (???:???)
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO - PID 5040 | #11: js::jit::MaybeEnterJit(JSContext*, js::RunState&) [js/src/jit/Jit.cpp:200]
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO - PID 5040 | #12: js::RunScript(JSContext*, js::RunState&) [js/src/vm/Interpreter.cpp:409]
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.245Z] 13:05:01 INFO - PID 5040 | #13: js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct, js::CallReason) [js/src/vm/Interpreter.cpp:580]
[task 2020-01-03T13:05:01.246Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.246Z] 13:05:01 INFO - PID 5040 | #14: InternalConstruct(JSContext*, js::AnyConstructArgs const&) [js/src/vm/Interpreter.cpp:655]
[task 2020-01-03T13:05:01.246Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.246Z] 13:05:01 INFO - PID 5040 | #15: js::Construct(JSContext*, JS::Handle<JS::Value>, js::AnyConstructArgs const&, JS::Handle<JS::Value>, JS::MutableHandle<JSObject >) [js/src/vm/Interpreter.cpp:710]
[task 2020-01-03T13:05:01.247Z] 13:05:01 INFO -
[task 2020-01-03T13:05:01.247Z] 13:05:01 INFO - PID 5040 | #16: JS::Construct(JSContext, JS::Handle<JS::Value>, JS::HandleValueArray const&, JS::MutableHandle<JSObject *>) [js/src/jsapi.cpp:2805]

Caused by the wpt test custom-elements/upgrading.html getting extended in bug 1599630 to disallow recursive cloning of custom elements.

Edgar, can you take care of this?