Teach SkyShell.apk to stop posting frames when not visible

sky/shell/ui/animator.h

Index: sky/shell/ui/animator.h
diff --git a/sky/shell/ui/animator.h b/sky/shell/ui/animator.h
index 15dd2f43f2ec8688de81e180ed1a62a5911b2ee8..cef6624cc6b8b52c91268578bbfdbb7e83ef789f 100644
--- a/sky/shell/ui/animator.h
+++ b/sky/shell/ui/animator.h
@@ -17,7 +17,9 @@ class Animator {
   ~Animator();
 
   void RequestFrame();
-  void CancelFrameRequest();
+
+  void Start();
+  void Stop();
 
  private:
   void BeginFrame();
@@ -27,6 +29,7 @@ class Animator {
   Engine* engine_;
   bool engine_requested_frame_;
   bool frame_in_progress_;
+  bool paused_;
 
   base::WeakPtrFactory<Animator> weak_factory_;