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This is the content layer wiring of getStats() with the receiver
selector argument. A follow up CL will add blink layer wiring and expose
it in JavaScript.
https://w3c.github.io/webrtc-pc/#dfn-stats-selection-algorithm

(This CL is very similar to the RTCRtpSender.getStats() version:
https://chromium-review.googlesource.com/c/chromium/src/+/973608)

Bug: 680172
Change-Id: I2f0cb0e997602c01ad6b1a2523663811c2e0ebe3
Reviewed-on: https://chromium-review.googlesource.com/975963
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Taylor Brandstetter <deadbeef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546760}