Add proposed restartIce() method to RTCPeerConnection behind a pref, to meet commitment.

There are some r+ patches which didn't land and no activity in this bug for 2 weeks.
:jib, could you have a look please?
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Since these patches contain wpt tests (which would get upstreamed), I'm holding off until a June 11th webrtc interim where this feature will be discussed.

Spec PR merged, so this can land without pref.

Would increasing the number of test chunks help with the fd issue?

https://searchfox.org/mozilla-central/source/taskcluster/ci/test/web-platform.yml#47

Added https://github.com/w3c/webrtc-pc/pull/2231 for latest change.

Try to increase number of test chunks.

Depending on what's opening files, you could also try adding "restart-after" to the metadata for some webrtc tests.

Docs:

A key ``restart-after`` which can be set to any value to indicate that
   the runner should restart the browser after running this test (e.g. to
   clear out unwanted state).

Bumping the number of chunks appears not to have worked (though there's no stack, so I've re-run it a few times to get one just in case):

[task 2019-07-17T03:37:26.321Z] 03:37:26     INFO - IOError on command, setting status to CRASH
[task 2019-07-17T03:37:26.538Z] 03:37:26     INFO - TEST-UNEXPECTED-CRASH | /webrtc/RTCPeerConnection-setRemoteDescription-rollback.html | expected OK

The newly added test is 10 tests prior to this crash. Will try a restart-after somewhere in the middle, thanks!

Trying restart-after.

:jib - when the time comes, please be sure to also add dev-doc-needed to any follow-up bugs around enabling this on various channels, or by default. Thanks!

This is already enabled, not behind a pref. Riding the trains.

https://hg.mozilla.org/mozilla-central/rev/7e047766fccd
https://hg.mozilla.org/mozilla-central/rev/1e8389cdefd3

Since the status are different for nightly and release, what's the status for beta?
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Documentation is up to date:

• RTCPeerConnection.restartIce()
• BCD update PR has been submitted to set the compatibility versions for Firefox and Chrome

Firefox 70 for developers was already updated by Chris.