Add a Clock and TickClock interface for mocking out time

media/base/pipeline.h

Index: media/base/pipeline.h
diff --git a/media/base/pipeline.h b/media/base/pipeline.h
index 8f976c9a010b75e5112169c6e9becf2208290f3c..4a24fd25fd702ed1f8777f717aafc5c2a1bde3d1 100644
--- a/media/base/pipeline.h
+++ b/media/base/pipeline.h
@@ -11,6 +11,7 @@
 #include "base/synchronization/condition_variable.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
+#include "base/time/default_clock.h"
 #include "media/base/audio_renderer.h"
 #include "media/base/demuxer.h"
 #include "media/base/media_export.h"
@@ -404,6 +405,9 @@ class MEDIA_EXPORT Pipeline
   // the filters.
   float playback_rate_;
 
+  // base::Clock used by |clock_|.
+  base::DefaultClock default_clock_;
+
   // Reference clock.  Keeps track of current playback time.  Uses system
   // clock and linear interpolation, but can have its time manually set
   // by filters.