cc: Make separate interface for BeginFrame ipc from OutputSurface

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 3b8a576c4ed9fa8363aeea15a2bdef772355234e..8086bed64bf787fb9387c6c7ed61c20ff26a59f3 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -10,6 +10,7 @@
 
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
+#include "base/memory/ref_counted.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/output/vsync_parameter_observer.h"
 #include "cc/scheduler/delay_based_time_source.h"
@@ -162,6 +163,27 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
   bool inside_as_value_into_;
 };
 
+// A frame source that delivers BeginFrame from outside of cc.
+// ExternalBeginFrameSource is ref-counted object because it could be used by
+// outside of cc to deliver new BeginFrame.
+class CC_EXPORT ExternalBeginFrameSource
+    : public BeginFrameSourceMixIn,
+      public base::RefCounted<ExternalBeginFrameSource>,
+      public NON_EXPORTED_BASE(base::NonThreadSafe) {
+ public:
+  ExternalBeginFrameSource() {}
+
+  // Tells the Source that client is ready to handle external BeginFrames.
+  virtual void SetClientReady() {}
+
+ protected:
+  friend class base::RefCounted<ExternalBeginFrameSource>;
+  virtual ~ExternalBeginFrameSource() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
+};
+
 // A frame source which calls BeginFrame (at the next possible time) as soon as
 // remaining frames reaches zero.
 class CC_EXPORT BackToBackBeginFrameSource : public BeginFrameSourceMixIn {