The failure was introduced due to a bug introduced in one of the tests in this PR: https://github.com/web-platform-tests/wpt/pull/12141

The addTrack(0 streams) test should not take any stream as argument to addTrack.

I'll create a fix, that is easier than reverting.

ugh... why didn't the tests fail on github?

I think because the failure only occurs when the test runs, which is when Unified Plan is enabled, which isn't the default. WPT bots don't know about that.

one more reason to land stuff via chrome then. Thank you!

Yes - better reviewing tools and better bots! Chrome bots don't run Firefox though.

Fix: https://chromium-review.googlesource.com/c/chromium/src/+/1155125

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e653e90ca6a59abbeaed25191c27b9a77a788b2b

commit e653e90ca6a59abbeaed25191c27b9a77a788b2b
Author: Henrik Boström <hbos@chromium.org>
Date: Tue Jul 31 08:06:52 2018

Fix RTCPeerConnection-transceivers.https.html bug.

This PR accidentally made the "addTrack(0 streams)" test call addTrack()
with a stream, which broke the tests:
https://github.com/web-platform-tests/wpt/pull/12141

Test restored and all "addTrack/addTransceiver(X streams)" are made to
consistently use "new MediaStream()" the same way.

Bug:  869036 
Change-Id: Ieaf0c73a1996aeb2e87e18f2f9065fa1e6fee1e7
Reviewed-on: https://chromium-review.googlesource.com/1155125
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579341}
[modify] https://crrev.com/e653e90ca6a59abbeaed25191c27b9a77a788b2b/third_party/WebKit/LayoutTests/external/wpt/webrtc/RTCPeerConnection-transceivers.https.html
[modify] https://crrev.com/e653e90ca6a59abbeaed25191c27b9a77a788b2b/third_party/WebKit/LayoutTests/virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/RTCPeerConnection-transceivers.https-expected.txt