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Added to webrtc rtpencoder struct is a field for specifying the dscp value
on a per-encoder basis, allowing audio and video to have different priorities.
The API is defined in https://w3c.github.io/webrtc-dscp-exp/ , and the
particular codepoints are defined in
https://tools.ietf.org/html/draft-ietf-tsvwg-rtcweb-qos-18#section-5
Note, actually enabling DSCP at all is still guarded by the googDscp
PeerConnection constraint, for now.

WPT tests:  https://chromium-review.googlesource.com/c/chromium/src/+/1357515
I2I: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/bVNynDp9PHo
chromestatus:  https://www.chromestatus.com/feature/5583278023114752


Bug: webrtc:5008
Change-Id: I05bdcf62787d30211310a4686bb5fc18fda674da
Reviewed-on: https://chromium-review.googlesource.com/c/1298231Reviewed-by: Florent Castelli <orphis@chromium.org>
Commit-Queue: Tim Haloun <thaloun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613218}