Implement googContentType GetStats metric reported on receive side.

webrtc/api/video/video_content_type.cc

Index: webrtc/api/video/video_content_type.cc
diff --git a/webrtc/api/video/video_content_type.cc b/webrtc/api/video/video_content_type.cc
index 278b57c5f5625b5dd909821215b017588ebf8588..351cfeefb600f1c189fe5167a47439c3079b8584 100644
--- a/webrtc/api/video/video_content_type.cc
+++ b/webrtc/api/video/video_content_type.cc
@@ -90,5 +90,9 @@ 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