Introduce webrtc::VideoFrame::timestamp_us, and corresponding constructor.

webrtc/media/engine/webrtcvideoengine2.h

Index: webrtc/media/engine/webrtcvideoengine2.h
diff --git a/webrtc/media/engine/webrtcvideoengine2.h b/webrtc/media/engine/webrtcvideoengine2.h
index 55da67adcde5e580522ea3b5f518ebb321535bad..807675f44bb3140733dc4b72b3c6e80cce4cdae6 100644
--- a/webrtc/media/engine/webrtcvideoengine2.h
+++ b/webrtc/media/engine/webrtcvideoengine2.h
@@ -406,13 +406,9 @@ class WebRtcVideoChannel2 : public VideoMediaChannel, public webrtc::Transport {
 
     bool sending_ GUARDED_BY(lock_);
 
-    // The timestamp of the first frame received
-    // Used to generate the timestamps of subsequent frames
-    rtc::Optional<int64_t> first_frame_timestamp_ms_ GUARDED_BY(lock_);
-
     // The timestamp of the last frame received
     // Used to generate timestamp for the black frame when source is removed
-    int64_t last_frame_timestamp_ms_ GUARDED_BY(lock_);
+    int64_t last_frame_timestamp_us_ GUARDED_BY(lock_);
   };
 
   // Wrapper for the receiver part, contains configs etc. that are needed to