Use microsecond timestamp in cricket::VideoFrame.

webrtc/media/engine/webrtcvideoengine2.h

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 377 matching lines @@
     VideoEncoderSettings encoder_settings_ GUARDED_BY(lock_);
     AllocatedEncoder allocated_encoder_ GUARDED_BY(lock_);
     Dimensions last_dimensions_ GUARDED_BY(lock_);
     webrtc::VideoRotation last_rotation_ GUARDED_BY(lock_) =
         webrtc::kVideoRotation_0;
 
     bool sending_ GUARDED_BY(lock_);
 
     // The timestamp of the first frame received
     // Used to generate the timestamps of subsequent frames
-    int64_t first_frame_timestamp_ms_ GUARDED_BY(lock_);
+    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_);
   };
 
   // Wrapper for the receiver part, contains configs etc. that are needed to
   // reconstruct the underlying VideoReceiveStream. Also serves as a wrapper
   // between rtc::VideoSinkInterface<webrtc::VideoFrame> and
   // rtc::VideoSinkInterface<cricket::VideoFrame>.
@@ skipping 124 matching lines @@
   // TODO(deadbeef): Don't duplicate information between
   // send_params/recv_params, rtp_extensions, options, etc.
   VideoSendParameters send_params_;
   VideoOptions default_send_options_;
   VideoRecvParameters recv_params_;
 };
 
 }  // namespace cricket
 
 #endif  // WEBRTC_MEDIA_ENGINE_WEBRTCVIDEOENGINE2_H_