Revert of Make SingleThreadProxy a SchedulerClient

cc/trees/layer_tree_host_single_thread_client.h

Index: cc/trees/layer_tree_host_single_thread_client.h
diff --git a/cc/trees/layer_tree_host_single_thread_client.h b/cc/trees/layer_tree_host_single_thread_client.h
index dbca28cdf25f809a764a670f06edf4d1c311132a..cb6145048e66aac79f4cf8eeb81ad88dc90ed66a 100644
--- a/cc/trees/layer_tree_host_single_thread_client.h
+++ b/cc/trees/layer_tree_host_single_thread_client.h
@@ -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