Teach SkyShell.apk to stop posting frames when not visible

sky/shell/ui/animator.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_SHELL_UI_ANIMATOR_H_
 #define SKY_SHELL_UI_ANIMATOR_H_
 
 #include "base/memory/weak_ptr.h"
 #include "sky/shell/ui/engine.h"
 
 namespace sky {
 namespace shell {
 
 class Animator {
  public:
   explicit Animator(const Engine::Config& config, Engine* engine);
   ~Animator();
 
   void RequestFrame();
-  void CancelFrameRequest();
+
+  void Start();
+  void Stop();
 
  private:
   void BeginFrame();
   void OnFrameComplete();
 
   Engine::Config config_;
   Engine* engine_;
   bool engine_requested_frame_;
   bool frame_in_progress_;
+  bool paused_;
 
   base::WeakPtrFactory<Animator> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Animator);
 };
 
 }  // namespace shell
 }  // namespace sky
 
 #endif  // SKY_SHELL_UI_ANIMATOR_H_