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..a74df7910bf9a8201f3d40819108c54dbc0da86b 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -162,6 +162,15 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
   bool inside_as_value_into_;
 };
 
+// A frame source that delivers BeginFrame from outside of CC layer.

[danakj, 2014/10/10 16:03:35]

from outside of cc.

[simonhong, 2014/10/15 01:04:21]

Done.

+class CC_EXPORT ExternalBeginFrameSource : public BeginFrameSourceMixIn {

[danakj, 2014/10/10 16:03:35]

This should inherit from BeginFrameSource (or why not just add SetClientReady to
BeginFrameSource and use that?)

The MixIn has concrete implementations, it should inherit from an abstract
class, not vice versa. And users of ExternalBFS shouldn't care that the MixIn
class is implementing parts of the interface.

[simonhong, 2014/10/15 01:04:21]

I think ExternalBeginFrameSource should have a different specs such as
ref-counted and non-thread safe object because it would be used to request
BeginFrame from outside of cc(in browser-side scheduler with unified
BeginFrame).
So, I made it as a concrete class for base class.

[danakj, 2014/10/15 06:49:49]

So you have a use case for why it would need to be ref counted?

+ public:
+  virtual ~ExternalBeginFrameSource() {}
+
+  // Tells the Source that client is ready to handle external BeginFrames.
+  virtual void SetClientReady() = 0;
+};
+
 // A frame source which calls BeginFrame (at the next possible time) as soon as
 // remaining frames reaches zero.
 class CC_EXPORT BackToBackBeginFrameSource : public BeginFrameSourceMixIn {