Add VideoProcessor interface.

The new VideoProcessor interface abstracts video layer in chromoting client.
RectangleUpdateDecoder is the default implementation. Another implementation
will be added to suppord decoding using <video> and MediaSource API (or
Pepper video decode API if we decide to use it instead in the future).

BUG=321825

Review URL: https://codereview.chromium.org/136763009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246547 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246547