Support alpha channel recording for VPX in MediaRecorder
This CL adds support for alpha channel recording in VpxEncoder in VideoTrackRecorder.
- OnEncodedVideoCB is modified to handle extraneous alpha channel output.
- VpxEncoder is refactored to use two encoder instances and encode alpha channel padded
with dummy data.
- WebmMuxer is modified to add the optional alpha channel data using AdditionalBlock.
BUG=690968
TEST=Added unit tests. Demo page on
https://cdn.rawgit.com/uysalere/js-demos/master/canvascapture_alpharecording.html.
Also, working on a alpha video based content_browsertest in a follow-up CL.
Review-Url: https://codereview.chromium.org/2691373005
Cr-Commit-Position: refs/heads/master@{#456331}
10 files changed
