Implement googContentType GetStats metric reported on receive side.

webrtc/api/video/video_content_type.h

 /*
  *  Copyright (c) 2017 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.
  */
 
 #ifndef WEBRTC_API_VIDEO_VIDEO_CONTENT_TYPE_H_
 #define WEBRTC_API_VIDEO_VIDEO_CONTENT_TYPE_H_
 
 #include <stdint.h>
 
+#include <string>
+
 namespace webrtc {
 
 enum class VideoContentType : uint8_t {
   UNSPECIFIED = 0,
   SCREENSHARE = 1,
 };
 
 namespace videocontenttypehelpers {
   bool SetExperimentId(VideoContentType* content_type,
                               uint8_t experiment_id);
   bool SetSimulcastId(VideoContentType* content_type,
                              uint8_t simulcast_id);
 
   uint8_t GetExperimentId(const VideoContentType& content_type);
   uint8_t GetSimulcastId(const VideoContentType& content_type);
 
   bool IsScreenshare(const VideoContentType& content_type);
 
   bool IsValidContentType(uint8_t value);
+
+  const char* ToString(const VideoContentType& content_type);
 }  // namespace videocontenttypehelpers
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_VIDEO_VIDEO_CONTENT_TYPE_H_