Decouple BrowserCompositorOutputSurface from BeginFrameSource.

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index bde01afd79ec6bd72a1d525c85b9d6d49bafe9d0..74655786ddda555121979c37d7ff2b58987a0566 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -16,6 +16,7 @@
 #include "base/trace_event/trace_event.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/scheduler/delay_based_time_source.h"
+#include "cc/scheduler/vsync_observer.h"
 
 namespace cc {
 
@@ -129,12 +130,11 @@ class CC_EXPORT StubBeginFrameSource : public BeginFrameSource {
 };
 
 // A frame source which ticks itself independently.
-class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource {
+class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource,
+                                            public VSyncObserver {
  public:
   ~SyntheticBeginFrameSource() override;
 
-  virtual void OnUpdateVSyncParameters(base::TimeTicks timebase,
-                                       base::TimeDelta interval) = 0;
   // This overrides any past or future interval from updating vsync parameters.
   virtual void SetAuthoritativeVSyncInterval(base::TimeDelta interval) = 0;
 };