Add metrics for tracking underflow during src= playback.

media/blink/webmediaplayer_impl.h

Index: media/blink/webmediaplayer_impl.h
diff --git a/media/blink/webmediaplayer_impl.h b/media/blink/webmediaplayer_impl.h
index 7c3da6389a8cadf684fc1fc93365e2a2f8730588..3cfa2b7237a4a5aec2802c236cf028b4612be76c 100644
--- a/media/blink/webmediaplayer_impl.h
+++ b/media/blink/webmediaplayer_impl.h
@@ -18,6 +18,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/thread.h"
+#include "base/timer/elapsed_timer.h"
 #include "base/timer/timer.h"
 #include "build/build_config.h"
 #include "media/base/media_tracks.h"
@@ -541,6 +542,10 @@ class MEDIA_BLINK_EXPORT WebMediaPlayerImpl
   std::unique_ptr<WatchTimeReporter> watch_time_reporter_;
   bool is_encrypted_;
 
+  // Number of times we've reached BUFFERING_HAVE_NOTHING during playback.
+  int underflow_count_;
+  std::unique_ptr<base::ElapsedTimer> underflow_timer_;
+
   DISALLOW_COPY_AND_ASSIGN(WebMediaPlayerImpl);
 };