Fix a time control bug, that the VCMReceiver::FrameForDecoding may over sleep.

Remark: a unit test to verify VCMReiceiver::FrameForDecoding will be in a separate CL.

BUG=4726
R=stefan@webrtc.org, wtc@chromium.org

Review URL: https://webrtc-codereview.appspot.com/53619004

Cr-Commit-Position: refs/heads/master@{#9362}
1 file changed