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This exposes RTCAudioSourceStats and RTCVideoSourceStats, which are the
RTCMediaSourceStats dictionaries, to JavaScript.

Implementation in WebRTC here:
https://webrtc-review.googlesource.com/c/src/+/137804/

This CL also re-enabled
WebRtcBrowserTest.RunsAudioVideoWebRTCCallInTwoTabsGetStatsPromise and
external/wpt/webrtc/RTCPeerConnection-track-stats.https.html
which were temporarily disabled due to blocking the WebRTC CL from
rolling.

Bug: webrtc:10453
Change-Id: I1c97a9798301c48437982fb656a1bdf5fbcabb36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621912
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663022}