Revert 280493 "Make SingleThreadProxy a SchedulerClient"

trunk/src/cc/trees/layer_tree_host_single_thread_client.h

Index: trunk/src/cc/trees/layer_tree_host_single_thread_client.h
===================================================================
--- trunk/src/cc/trees/layer_tree_host_single_thread_client.h	(revision 280520)
+++ trunk/src/cc/trees/layer_tree_host_single_thread_client.h	(working copy)
@@ -10,10 +10,10 @@
 class LayerTreeHostSingleThreadClient {
  public:
   // Request that the client schedule a composite.
-  virtual void ScheduleComposite() {}
+  virtual void ScheduleComposite() = 0;
   // Request that the client schedule a composite now, and calculate appropriate
   // delay for potential future frame.
-  virtual void ScheduleAnimation() {}
+  virtual void ScheduleAnimation() = 0;
 
   // Called whenever the compositor posts a SwapBuffers (either full or
   // partial). After DidPostSwapBuffers(), exactly one of