Add watch time metrics for HTML5 media playback.

media/base/media_log_event.h

Index: media/base/media_log_event.h
diff --git a/media/base/media_log_event.h b/media/base/media_log_event.h
index a5427fbd4b87bb5f99a24546c12b8adb520e052f..73d81dcaf1da192770a812c8c12ce40d32e4663a 100644
--- a/media/base/media_log_event.h
+++ b/media/base/media_log_event.h
@@ -95,7 +95,10 @@ struct MediaLogEvent {
     // A property has changed without any special event occurring.
     PROPERTY_CHANGE,
 
-    TYPE_LAST = PROPERTY_CHANGE
+    // Indicates that updated watch time statistics are available.
+    WATCH_TIME_UPDATE,
+
+    TYPE_LAST = WATCH_TIME_UPDATE
   };
 
   int32_t id;