Implement googContentType GetStats metric reported on receive side.

webrtc/api/video/video_content_type.h

Index: webrtc/api/video/video_content_type.h
diff --git a/webrtc/api/video/video_content_type.h b/webrtc/api/video/video_content_type.h
index 6f6bbe8e43971ee3df4ab08e3815f4f1d937b1db..8addb13851f1f35339cf22dd8b90b3f880f6e48f 100644
--- a/webrtc/api/video/video_content_type.h
+++ b/webrtc/api/video/video_content_type.h
@@ -13,6 +13,8 @@
 
 #include <stdint.h>
 
+#include <string>
+
 namespace webrtc {
 
 enum class VideoContentType : uint8_t {
@@ -32,6 +34,8 @@ namespace videocontenttypehelpers {
   bool IsScreenshare(const VideoContentType& content_type);
 
   bool IsValidContentType(uint8_t value);
+
+  const char* ToString(const VideoContentType& content_type);
 }  // namespace videocontenttypehelpers
 
 }  // namespace webrtc