Thread-safe ChannelManager.GetSupportedFormats, used by VideoSource

VideoSource was using VideoCapturer's GetSupportedFormats in a non-thread safe manner.
Now this is handled to (new method) ChannelManager.GetSupportedFormats.

BUG=
R=perkj@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42079004

Cr-Commit-Position: refs/heads/master@{#8495}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8495 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed