[Media] Record time it takes to start rendering audio and video

media/renderers/video_renderer_impl.h

Index: media/renderers/video_renderer_impl.h
diff --git a/media/renderers/video_renderer_impl.h b/media/renderers/video_renderer_impl.h
index 3ca8cc57d4688b8a20c3675eb6d5d7bb68547368..8eaca2513474952ab117b9818b446471b97b4b99 100644
--- a/media/renderers/video_renderer_impl.h
+++ b/media/renderers/video_renderer_impl.h
@@ -17,6 +17,7 @@
 #include "base/memory/weak_ptr.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/synchronization/lock.h"
+#include "base/time/time.h"
 #include "base/timer/timer.h"
 #include "media/base/decryptor.h"
 #include "media/base/demuxer_stream.h"
@@ -289,6 +290,11 @@ class MEDIA_EXPORT VideoRendererImpl
   // Indicates if we've painted the first valid frame after StartPlayingFrom().
   bool painted_first_frame_;
 
+  // The timestamp recorded from |tick_clock_| when the renderer switches from
+  // rendering in the background to rendering in the foreground.
+  // Used to compute how much time it took to paint the first frame.
+  base::TimeTicks shown_timestamp_;
+
   // NOTE: Weak pointers must be invalidated before all other member variables.
   base::WeakPtrFactory<VideoRendererImpl> weak_factory_;