The browser toolbox often fails opening with the following exception:
SecurityError: Permission denied to access property "windowUtils" on cross-origin object

It looks like this code:
https://searchfox.org/mozilla-beta/rev/52cc26c5eccd4f496ab8a8c73b529e4f0e23e4bd/devtools/server/actors/utils/event-loop.js#117-124

 this._thread.dbg.getDebuggees()

Sometimes also return window often for remote iframes.
These window objects are special and most property access will throw.
In addition to that, we should ignore these windows, as they will be debugged via another thread actor, running in the process of this remote iframe.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=664e2b662b0799f39e8a6d0422fab74d68568cad

Backed out changeset 9d95edbd8386 (Bug 1660892) for causing devtool timeouts.

Backout link: https://hg.mozilla.org/integration/autoland/rev/7cea08d78f8b23d7c8a41a76c9bb90c69df3a9bf

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception%2Csuccess%2Cusercancel%2Crunning%2Cpending%2Crunnable&searchStr=linux%2C18.04%2Cx64%2Copt%2Cmochitests%2Ctest-linux1804-64%2Fopt-mochitest-devtools-chrome-e10s%2Cdt1&tochange=9d95edbd8386f1271b641fbcea1ffb4def211258&fromchange=1415fd1b5052c4150f9df667ec9e7c34f3624917

https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception%2Csuccess%2Cusercancel%2Crunning%2Cpending%2Crunnable&fromchange=d1a42f86dd6125fe9dd6a3e56e5b13972821391c&searchStr=os%2Cx%2C10.14%2Copt%2Cmochitests%2Ctest-macosx1014-64%2Fopt-mochitest-devtools-chrome-e10s%2Cdt3&tochange=9d95edbd8386f1271b641fbcea1ffb4def211258&selectedTaskRun=FMlWFnC9SyWvjf5pDavFcg.0

Failure log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=314079274&repo=autoland&lineNumber=16655
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=314077215&repo=autoland&lineNumber=16234

Oops, I thouht this failure was related to the next patch of this queue, which I explicitely avoided to land.

Let me see if I fixed this issue now:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=21418f9e2d725bc267c36a28b4940c3a2150c290

https://hg.mozilla.org/mozilla-central/rev/28f5c2767938