WebRtcVideoCapturer: Getting rid of the |critical_section_stopping_| lock and all of its critical sections.

This avoids a deadlock in WebRtcVideoCapturer.
The deadlock could occur because OnIncomingFrame() has the |critical_section_stopping_| lock, which could block a Stop() on the |start_thread_|. When OnIncomingFrame() then tries to do synchronous invoke on |start_thread_| (before releasing said lock) we have a deadlock.

BUG=4670
R=magjed@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47259004

Cr-Commit-Position: refs/heads/master@{#9294}
2 files changed