Splitting the work that needs to be done for bug 1322939.

This is the first part of it.

This patch completes the top layer requirement for showModal()
Spec: https://html.spec.whatwg.org/multipage/interactive-elements.html#dom-dialog-showmodal

Backed out changeset bb7c330d407f (bug 1637307) for wpt failures at layout/generic/ViewportFrame.cpp

Backout: https://hg.mozilla.org/integration/autoland/rev/462b79772df2f4baa8bd5a537c43c92d0cb2a542

Failure push: https://treeherder.mozilla.org/#/jobs?repo=autoland&revision=bb7c330d407f8d233b485600ffc59a06ef8e797f

Failure log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=302933809&repo=autoland&lineNumber=38446

task 2020-05-19T17:40:36.915Z] 17:40:36 INFO - TEST-START | /html/semantics/embedded-content/the-canvas-element/toBlob.null.html
[task 2020-05-19T17:40:36.917Z] 17:40:36 INFO - Closing window 770
[task 2020-05-19T17:40:37.028Z] 17:40:37 INFO - PID 8812 | [Child 5464, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80004005 (NS_ERROR_FAILURE): file /builds/worker/checkouts/gecko/netwerk/base/nsNetUtil.cpp, line 2295
[task 2020-05-19T17:40:37.028Z] 17:40:37 INFO - PID 8812 | [Child 5464, Main Thread] WARNING: 'NS_FAILED(rv)', file /builds/worker/checkouts/gecko/caps/ContentPrincipal.cpp, line 398
[task 2020-05-19T17:40:37.028Z] 17:40:37 INFO - PID 8812 | [Child 5464, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80004005 (NS_ERROR_FAILURE): file /builds/worker/checkouts/gecko/caps/ContentPrincipal.cpp, line 423
[task 2020-05-19T17:40:37.054Z] 17:40:37 INFO - PID 8812 | [Child 5464, Main Thread] WARNING: Extra shutdown CC: 'i < NORMAL_SHUTDOWN_COLLECTIONS', file /builds/worker/checkouts/gecko/xpcom/base/nsCycleCollector.cpp, line 3359
[task 2020-05-19T17:40:37.194Z] 17:40:37 INFO - PID 8812 | ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to c:\users\task_1589906784\appdata\local\temp\tmpqbefbf\runtests_leaks_5716_tab_pid8420.log
[task 2020-05-19T17:40:37.195Z] 17:40:37 INFO - PID 8812 | [8420, Main Thread] WARNING: XPCOM_MEM_BLOAT_LOG is set, disabling native allocations.: file /builds/worker/checkouts/gecko/tools/profiler/core/platform.cpp, line 224
[task 2020-05-19T17:40:37.253Z] 17:40:37 INFO - PID 12476 | ### XPCOM_MEM_BLOAT_LOG defined -- logging bloat/leaks to c:\users\task_1589906784\appdata\local\temp\tmpkko7wy\runtests_leaks_5716_tab_pid3024.log
[task 2020-05-19T17:40:37.254Z] 17:40:37 INFO - PID 12476 | [3024, Main Thread] WARNING: XPCOM_MEM_BLOAT_LOG is set, disabling native allocations.: file /builds/worker/checkouts/gecko/tools/profiler/core/platform.cpp, line 224
[task 2020-05-19T17:40:37.254Z] 17:40:37 INFO - PID 12476 | [Child 12560, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80004005 (NS_ERROR_FAILURE): file /builds/worker/checkouts/gecko/netwerk/base/nsNetUtil.cpp, line 2295
[task 2020-05-19T17:40:37.255Z] 17:40:37 INFO - PID 12476 | [Child 12560, Main Thread] WARNING: 'NS_FAILED(rv)', file /builds/worker/checkouts/gecko/caps/ContentPrincipal.cpp, line 398
[task 2020-05-19T17:40:37.255Z] 17:40:37 INFO - PID 12476 | [Child 12560, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80004005 (NS_ERROR_FAILURE): file /builds/worker/checkouts/gecko/caps/ContentPrincipal.cpp, line 423
[task 2020-05-19T17:40:37.294Z] 17:40:37 INFO - PID 12476 | [Child 12560, Main Thread] WARNING: Extra shutdown CC: 'i < NORMAL_SHUTDOWN_COLLECTIONS', file /builds/worker/checkouts/gecko/xpcom/base/nsCycleCollector.cpp, line 3359
[task 2020-05-19T17:40:37.365Z] 17:40:37 INFO - PID 8812 | Couldn't convert chrome URL: chrome://branding/locale/brand.properties
[task 2020-05-19T17:40:37.365Z] 17:40:37 INFO - PID 8812 | [Child 8420, Main Thread] WARNING: Could not get the program name for a cubeb stream.: 'NS_SUCCEEDED(rv)', file /builds/worker/checkouts/gecko/dom/media/CubebUtils.cpp, line 375
[task 2020-05-19T17:40:37.610Z] 17:40:37 INFO - PID 8812 | Assertion failure: !aBuilder->IsForPainting() || !ShouldInTopLayerForFullscreen(elem), at /builds/worker/checkouts/gecko/layout/generic/ViewportFrame.cpp:131
[task 2020-05-19T17:40:37.678Z] 17:40:37 INFO - .

https://hg.mozilla.org/mozilla-central/rev/fcfe734c8031