Adding UMA on page load size for pages with Media

chrome/browser/page_load_metrics/page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_observer.h b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
index 80600dbb784babe703b344c0287c853d06843ed0..41acd9cd0f165b1e0b712c63f3a74a0f2e658d18 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
@@ -11,6 +11,7 @@
 #include "base/optional.h"
 #include "chrome/common/page_load_metrics/page_load_timing.h"
 #include "content/public/browser/navigation_handle.h"
+#include "content/public/browser/web_contents_observer.h"
 #include "third_party/WebKit/public/platform/WebInputEvent.h"
 #include "url/gurl.h"
 
@@ -323,6 +324,11 @@ class PageLoadMetricsObserver {
   virtual void OnLoadingBehaviorObserved(
       const page_load_metrics::PageLoadExtraInfo& extra_info) {}
 
+  // Invoked when a media element starts playing.
+  virtual void MediaStartedPlaying(
+      const content::WebContentsObserver::MediaPlayerInfo& video_type,
+      bool is_in_main_frame) {}
+
   // Invoked when the UMA metrics subsystem is persisting metrics as the
   // application goes into the background, on platforms where the browser
   // process may be killed after backgrounding (Android). Implementers should