Adding UMA on page load size for pages with Media

chrome/browser/page_load_metrics/page_load_tracker.h

Index: chrome/browser/page_load_metrics/page_load_tracker.h
diff --git a/chrome/browser/page_load_metrics/page_load_tracker.h b/chrome/browser/page_load_metrics/page_load_tracker.h
index bbd86413306d7dd9d6204b5e9ed029fe104ed1bb..08111a55442c9e811c7914bb8bb04e7fabdaea0e 100644
--- a/chrome/browser/page_load_metrics/page_load_tracker.h
+++ b/chrome/browser/page_load_metrics/page_load_tracker.h
@@ -15,6 +15,7 @@
 #include "chrome/browser/page_load_metrics/user_input_tracker.h"
 #include "chrome/common/page_load_metrics/page_load_timing.h"
 #include "content/public/browser/global_request_id.h"
+#include "content/public/browser/web_contents_observer.h"
 #include "ui/base/page_transition_types.h"
 
 class GURL;
@@ -231,6 +232,11 @@ class PageLoadTracker {
   bool HasMatchingNavigationRequestID(
       const content::GlobalRequestID& request_id) const;
 
+  // Invoked when a media element starts playing.
+  void MediaStartedPlaying(
+      const content::WebContentsObserver::MediaPlayerInfo& video_type,
+      bool is_in_main_frame);
+
  private:
   // This function converts a TimeTicks value taken in the browser process
   // to navigation_start_ if: