Add support for content hints to VideoTrack.

webrtc/api/rtpsender.h

 /*
  *  Copyright 2015 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 207 matching lines @@
   void SetVideoSend();
   // Helper function to call SetVideoSend with "stop sending" parameters.
   void ClearVideoSend();
 
   std::string id_;
   std::string stream_id_;
   cricket::VideoChannel* channel_ = nullptr;
   rtc::scoped_refptr<VideoTrackInterface> track_;
   uint32_t ssrc_ = 0;
   bool cached_track_enabled_ = false;
+  VideoTrackInterface::ContentHint cached_track_content_hint_ =
+      VideoTrackInterface::ContentHint::kNone;
   bool stopped_ = false;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_RTPSENDER_H_