Change so that capture_time_ms == 0 is considered invalid.
Change so that capture_time_ms == 0 is considered invalid.

No transmission offset computed if capture_time_ms is less than or equal to zero.

BUG=

Review URL: https://webrtc-codereview.appspot.com/761004

git-svn-id: 9b397c5dfe384c04b1b200d5305b653241000171@2675 4adac7df-926f-26a2-2b94-8c16560cd09d

lukeweber/webrtc-src-override,lukeweber/webrtc-src-override,lukeweber/webrtc-src-override,lukeweber/webrtc-src-override,lukeweber/webrtc-src-override,lukeweber/webrtc-src-override,lukeweber/webrtc-src-override