Screen Video Capture: Implement suspend optimization.

Implements frame delivery suspend/resume functionality in tab capture,
Aura window/desktop capture, and Android screen capture. This allows the
screen capture devices to minimize resource utilization while there are
no consumers of the video frames attached downstream.

Prerequisite change: https://codereview.chromium.org/2365223002/

BUG=650113

Review-Url: https://codereview.chromium.org/2364413002
Cr-Commit-Position: refs/heads/master@{#421763}
8 files changed