Add watch time metrics for HTML5 media playback.

media/base/media_log.h

Index: media/base/media_log.h
diff --git a/media/base/media_log.h b/media/base/media_log.h
index 64a3f3d4c86578fe5ade5070f883edeb27f7b955..cfcf05308925ca59373f6a8646e6394c609563c6 100644
--- a/media/base/media_log.h
+++ b/media/base/media_log.h
@@ -83,6 +83,18 @@ class MEDIA_EXPORT MediaLog : public base::RefCountedThreadSafe<MediaLog> {
   void SetDoubleProperty(const std::string& key, double value);
   void SetBooleanProperty(const std::string& key, bool value);
 
+  // Histogram names used for reporting; also double as MediaLog key names.
+  static const char kWatchTimeAudioVideoAll[];
+  static const char kWatchTimeAudioVideoMse[];
+  static const char kWatchTimeAudioVideoEme[];
+  static const char kWatchTimeAudioVideoSrc[];
+  static const char kWatchTimeAudioVideoBattery[];
+  static const char kWatchTimeAudioVideoAc[];
+
+  // Markers which signify the watch time should be finalized immediately.
+  static const char kWatchTimeFinalize[];
+  static const char kWatchTimeFinalizePower[];
+
  protected:
   friend class base::RefCountedThreadSafe<MediaLog>;
   virtual ~MediaLog();