FrameBuffer2 now has Start/Stop methods.

The Stop method is used to signal any thread that is waiting in the
NextFrame function and will cause it to return immediately.

BUG=webrtc:5514
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2105323002 .

Cr-Original-Commit-Position: refs/heads/master@{#13349}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 504c47d750b4516ca81c93ffb637d9a4c8fb6ea9