cc:: Change plumbing for external_begin_frame_source to the Scheduler.

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index 132a4ff62d5fda0d0a0660ee769b13b4e26f235a..cec45e743b960906ee449de386c19e3f2695e79f 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -24,6 +24,7 @@ class Vector2d;
 }
 
 namespace cc {
+class BeginFrameSource;
 class LayerTreeDebugState;
 class OutputSurface;
 struct RendererCapabilities;
@@ -69,7 +70,8 @@ class CC_EXPORT Proxy {
   virtual bool BeginMainFrameRequested() const = 0;
 
   // Must be called before using the proxy.
-  virtual void Start() = 0;
+  virtual void Start(
+      scoped_ptr<BeginFrameSource> external_begin_frame_source) = 0;
   virtual void Stop() = 0;   // Must be called before deleting the proxy.
 
   virtual bool SupportsImplScrolling() const = 0;