Allow RTCPeerConnection-track-stats.https.html to fail.

This unblocks a third_party/webrtc roll which adds new metrics. Making
this test pass again is blocked on
https://chromium-review.googlesource.com/c/chromium/src/+/1621912 but
that CL is blocked on the roll. To get out of this situation, the test
is temporarily allowed to both pass and fail.

TBR=guidou@chromium.org
NOTRY=True

Bug: webrtc:10453
Change-Id: I5d2986d642efa92bb93a9e008956655e03967fc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627039Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Auto-Submit: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662643}

third_party/blink/web_tests/TestExpectations

@@ -5681,3 +5681,7 @@ crbug.com/965389 [ Mac ] media/track/track-cue-rendering-position-auto.html [ Pa
 
 # Sheriff 2019-05-21
 crbug.com/965134 [ Linux ] virtual/mouseevent_fractional/fast/events/pointerevents/multi-touch-events.html [ Failure ]
+
+# Temporarily allow failing to unblock third_party/webrtc from rolling.
+crbug.com/webrtc/10453 external/wpt/webrtc/RTCPeerConnection-track-stats.https.html [ Pass Failure ]
+crbug.com/webrtc/10453 virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCPeerConnection-track-stats.https.html [ Pass Failure ]