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 3d849a40d638708ad37acca859aaeaa98e9e911e..e96a3e7fa558745e40fff78921f805108e06183c 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -117,6 +117,9 @@ class CC_EXPORT BeginFrameSource {
   virtual void AddObserver(BeginFrameObserver* obs) = 0;
   virtual void RemoveObserver(BeginFrameObserver* obs) = 0;
 
+  // Tells the Source that client is ready to handle BeginFrames messages.
+  virtual void SetClientReady() = 0;

[brianderson, 2014/10/23 02:12:44]

Anyway we can avoid having this method as part of the BeginFrameSource
interface? It's only needed for external sources to setup message routing. Is
there a different hook we can use?

[simonhong, 2014/10/24 14:30:18]

At first, I created ExternalBeginFrameSource deriving BeginFrameSourceMixIn.
It has SetClientReady() as virtual abstract method.
But, danakj@ pointed out that BeginFrameSourceMixIn should not be a base class
of abstract class.
Instead, danakj@ suggested adding SetClientReady() into BeginFrameSource.

Other option is adding SetClientReady() into BFSMixIn.
But I think all of Source inherits BFSMixIn so, it is same as current.

In this case, what is the best solution? :)

[brianderson, 2014/10/24 20:46:58]

Ok, what you have now is good then. Any other solution I can think of is going
to add more boilerplate than is really worth it.

[simonhong, 2014/10/29 14:47:13]

Yep!

+
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.
   virtual void AsValueInto(base::debug::TracedValue* dict) const = 0;
@@ -139,6 +142,7 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
   void DidFinishFrame(size_t remaining_frames) override {}
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
+  void SetClientReady() override {}

[brianderson, 2014/10/23 02:12:44]

Delete this line so we do not override SetClientReady() and all subclasses must
implement it.

The other implementations above are expected to be the same for all subclasses,
which is why they aren't virtual.

[brianderson, 2014/10/24 20:46:58]

Ignore my comment here. I didn't know that override implies virtual before, but
I do now. Also, inheriting an empty implementation should be fine in this case.

It'll probably be a good idea to make NeedsBeginFrames, SetNeedsBeginFrames,
AddObserver, and RemoveObserver "final" though since MixIn subclasses shouldn't
override them.

[simonhong, 2014/10/29 14:47:12]

Some of them are still used by its subclass.
AddObserver & RemoveObserver only can be final candidate. 

[brianderson, 2014/11/01 01:08:32]

Hmm, NeedsBeginFrames and SetNeedsBeginFrames should also be able to be final
since BeginFrameSourceMixIn has OnNeedsBeginFramesChange that it expects
subclasses to implement. Don't worry about cleaning it up in this patch though,
I've opened crbug.com/429391 for followup.

 
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.