MediaRecorderHandler: connecting Blink factory method to content/

MediaRecorderHandler changes to implementing WebMediaRecorderHandler;

content/ overrides Blink's createMediaRecorderHandler().

context: MediaRecorderHandler is the Blink-orchestrated class
implementing MediaStream Recorder API (see below). It plugs
together an existing MediaStreamVideoTrack to a new
VideoTrackRecorder-WebmMuxer pair. When MSVTrack passes
frames, these get encoded, muxed, and the result is sent to
Blink. See DD @ https://goo.gl/vSjzC5 (*) for the plan. The
previous CL http://crrev.com/1313603004 landed
MediaRecorderHandler, this one allows for its activation.

(*) Used to be https://goo.gl/kreaQj

Depends on https://crrev.com/1319473016 to disable
Blink tests, that after landing-rolling, should be updated and
reenabled.

BUG=262211

Review URL: https://codereview.chromium.org/1330873002

Cr-Commit-Position: refs/heads/master@{#348338}
6 files changed