Hook up ui::Compositor to Display's BeginFrameSource

cc/surfaces/display_client.h

Index: cc/surfaces/display_client.h
diff --git a/cc/surfaces/display_client.h b/cc/surfaces/display_client.h
index 61ea4d39970e79678e433eb8da6a784bdd5fe4c5..ff9c43741e5b9ded064c611784b7f68a8db85442 100644
--- a/cc/surfaces/display_client.h
+++ b/cc/surfaces/display_client.h
@@ -10,6 +10,7 @@
 
 namespace cc {
 
+class BeginFrameSource;
 struct ManagedMemoryPolicy;
 
 class DisplayClient {
@@ -18,6 +19,8 @@ class DisplayClient {
                                      base::TimeDelta interval) = 0;
   virtual void OutputSurfaceLost() = 0;
   virtual void SetMemoryPolicy(const ManagedMemoryPolicy& policy) = 0;
+  // This must be non-null and always the same.

[enne (OOO), 2016/03/22 21:05:18]

This has to always the same because the SurfaceDisplayOutputSurface just pulls
this value and has no way of knowing if its changes.

+  virtual BeginFrameSource* BeginFrameSourceForChildren() = 0;
 
  protected:
   virtual ~DisplayClient() {}