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This CL introduces MediaStreamTrackProcessor, an object that allows
exposing a MediaStreamTrack's media frames as a readable stream,
enabling applications to perform custom processing.
This basically allows the creation of a custom sink, in terms of the
model proposed by the Media Capture and Streams spec [1].
A MediaStreamTrackProcessor can be combined with a TransformStream and a
MediaStreamTrackGenerator to effectively insert a processing step inside
an existing track.

[1] https://w3c.github.io/mediacapture-main/#the-model-sources-sinks-constraints-and-settings

Bug: 1142955
Change-Id: I2a119f0c2ea9ce50432509286bd0f43758d8a272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524322
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826153}