Implement googContentType GetStats metric reported on receive side.

webrtc/api/video/video_content_type.cc

 /*
  *  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.
  */
 
@@ skipping 72 matching lines @@
 
 bool IsScreenshare(
     const VideoContentType& content_type) {
   return (static_cast<uint8_t>(content_type) & kScreenshareBitsMask) > 0;
 }
 
 bool IsValidContentType(uint8_t value) {
   // Any 6-bit value is allowed.
   return value < (1 << kTotalBitsSize);
 }
+
+const char* ToString(const VideoContentType& content_type) {
+  return IsScreenshare(content_type) ? "screen" : "realtime";
+}
 }  // namespace videocontenttypehelpers
 }  // namespace webrtc