Fix thread safety in VcmCapturer.

Makes VcmCapturer::Stop() blocking so that no frames can be in delivery
while the camera has stopped.

BUG=
R=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1411813004 .

Cr-Original-Commit-Position: refs/heads/master@{#10385}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1e737c6f2cf835a5b38dab0fc989ab52a749c87a
2 files changed