Modify video_render/ to allow a single old frame.

This stabilizes tests as a single frame reaches end-to-end, as well as
allowing slow or heavily-loaded systems to see any video updates even if
the frame takes more than 500ms in the pipeline.

R=mflodman@webrtc.org, stefan@webrtc.org
BUG=2724

Review URL: https://webrtc-codereview.appspot.com/5949004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5303 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed