MediaStream: Adding VideoTrackRecorder class and unittests

VTR is in charge of grabbing VideoFrames, encoding them on
IO thread and sending them forward to the registered client
(which will be in time a WebmMuxer crrev.com/1225123006).

See DD @ https://goo.gl/vSjzC5 (*) for the plan and
https://codereview.chromium.org/1211973012/ for a
hack of all Chrome parts.

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

BUG=262211

Review URL: https://codereview.chromium.org/1233033002

Cr-Commit-Position: refs/heads/master@{#345437}
9 files changed