Add a new VideoTrackSink interface.
It expose an interface to content/public that can be used for receiving video frames from a VideoTrack.
As a proof of concept this cl change the existing RtcVideoRenderer to implement this VideoTrackSink interface. This use of VideoTrackSink shows that an implementation can  get video frames from both local and remote video tracks.

The actual VideoTrack implementation still use the existing webrtc interface based tracks.

This is a Piranha Plant cl where the end goal is to minimize the use of libjingle interfaces in the MediaStream implementation.
Design doc: https://goto.google.com/qrbzy

BUG= 323223

Review URL: https://codereview.chromium.org/83023005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238694 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238694