Add feature to disallow 3rd party origin subframe JS dialogs

[chromium-wpt-export-bot]

Test: Covered by WPT, except for alert (which can't be tested in WPT).

Bug: 1065085
Change-Id: I8a58be63014c7893ab864d3582516a9eddbf5e25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2645693
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#851920}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[carlosjoan91]

@web-platform-tests/wpt-core-team Re:wpt-chrome-dev-stability. Chrome is expected to fail this WPT since the feature is not still implemented in Chrome by default. But this is triggering the flakiness check due to it sometimes failing (due to an unexpected alert), and other times timing out (due to the alert remaining open presumably).
My guess is increasing the timeout would always result in a failure and get rid of the flakiness, but it is unclear to me how to do that, or what would be the best way to proceed here. (This is temporary regardless, as once the feature is enabled in Chrome, it will pass the test instead of flaking between timeout and error).

[domenic]

@carlosjoan91 you can add <meta timeout="long"> to the top to increase the timeout. However, if that doesn't work, or you think it's not a good idea for long-term maintenance, then we can just get @web-platform-tests/wpt-core-team to admin-merge this despite the temporary flakiness.

[stephenmcgruer]

Ack; I suspect this is actually showing a bug with how we handle unexpectedly open alerts (possibly in webdriver). I don't think a long timeout would help.

We're happy to admin-merge this PR once the Chromium CL has landed.

[carlosjoan91]

Thanks for merging! And yeah, having this in the current flaky between failing and timing out is not particularly terrible, since it will go back to reliably passing once the feature is enabled by default in Chromium.