Make media::WallClockTimeSource thread safe.

media/base/wall_clock_time_source.h

Index: media/base/wall_clock_time_source.h
diff --git a/media/base/wall_clock_time_source.h b/media/base/wall_clock_time_source.h
index 87e28af88469153516858b372bdacd1c8d6d36f4..8057746afce0e836de03302d4b8164671570c438 100644
--- a/media/base/wall_clock_time_source.h
+++ b/media/base/wall_clock_time_source.h
@@ -6,6 +6,7 @@
 #define MEDIA_BASE_WALL_CLOCK_TIME_SOURCE_H_
 
 #include "base/memory/scoped_ptr.h"
+#include "base/synchronization/lock.h"
 #include "media/base/media_export.h"
 #include "media/base/time_source.h"
 
@@ -32,6 +33,8 @@ class MEDIA_EXPORT WallClockTimeSource : public TimeSource {
   void SetTickClockForTesting(scoped_ptr<base::TickClock> tick_clock);
 
  private:
+  base::TimeDelta CurrentMediaTime_Locked();
+
   scoped_ptr<base::TickClock> tick_clock_;
   bool ticking_;
 
@@ -42,6 +45,10 @@ class MEDIA_EXPORT WallClockTimeSource : public TimeSource {
   base::TimeDelta base_time_;
   base::TimeTicks reference_wall_ticks_;
 
+  // TODO(scherkus): Remove internal locking from this class after access to
+  // Renderer::CurrentMediaTime() is single threaded http://crbug.com/370634
+  base::Lock lock_;
+
   DISALLOW_COPY_AND_ASSIGN(WallClockTimeSource);
 };