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This CL makes the Chromium HW video factories implement the new
interfaces directly. It also adds SW codecs, simulcast, and SW fallback
codecs manually, instead of relying on WebRTC to do this. This will
give Chromium more flexibility in the future, and will allow us to
simplify the WebRTC code, since it's only Chromium that uses some of
these features.

Bug: webrtc:7925
Change-Id: I2bbd8b9a8c52bfe963d601f34188faf47d59e67e
Reviewed-on: https://chromium-review.googlesource.com/788410Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Commit-Queue: Magnus Jedvert <magjed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520062}