Implement timestamp translation/filter in VideoCapturer.

webrtc/media/engine/webrtcvideoframefactory.cc

Index: webrtc/media/engine/webrtcvideoframefactory.cc
diff --git a/webrtc/media/engine/webrtcvideoframefactory.cc b/webrtc/media/engine/webrtcvideoframefactory.cc
index 2c08c63129c6c28303a743d5122bf69d6e0f57d3..17c985f595a73ca3c544b0288abc0a9b2a0b4590 100644
--- a/webrtc/media/engine/webrtcvideoframefactory.cc
+++ b/webrtc/media/engine/webrtcvideoframefactory.cc
@@ -24,6 +24,7 @@ VideoFrame* WebRtcVideoFrameFactory::CreateAliasedFrame(
         "Failed to create WebRtcVideoFrame in CreateAliasedFrame.";
     return NULL;
   }
+  frame->set_timestamp_us(frame->timestamp_us() + timestamp_offset_us_);
   return frame.release();
 }