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 12371b4e3b511e7233cbfe5746b2110d7a9684a8..cd50b92f04f7dfee2b65e7fa43a20ec406bdad06 100644
--- a/media/base/pipeline.h
+++ b/media/base/pipeline.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/default_clock.h"
 #include "base/gtest_prod_util.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/synchronization/lock.h"
@@ -400,6 +401,9 @@ 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.