* First do all frame processing, then do all rate control
statistics calculations. This means that we only need to
synchronize once.
* Run the VideoProcessor on a task queue, thus supporting Android
HW codecs.
* Add some unit tests for the VideoProcessor.
BUG=webrtc:6634
Review-Url: https://codereview.webrtc.org/2997283002
Cr-Commit-Position: refs/heads/master@{#19604}