Teach SkyShell.apk to stop posting frames when not visible

sky/shell/ui/engine.h

Index: sky/shell/ui/engine.h
diff --git a/sky/shell/ui/engine.h b/sky/shell/ui/engine.h
index f4c51ffac368fc74836e4d2b62fb664e1951d196..fbda1ec449a5e9afbf87fcdc7970ac6fe6a75dce 100644
--- a/sky/shell/ui/engine.h
+++ b/sky/shell/ui/engine.h
@@ -71,6 +71,9 @@ class Engine : public UIDelegate,
   void RunFromSnapshot(const mojo::String& path) override;
   void RunFromBundle(const mojo::String& path) override;
 
+  void OnActivityPaused() override;
+  void OnActivityResumed() override;
+
   // SkyViewClient methods:
   void ScheduleFrame() override;
   void DidCreateIsolate(Dart_Isolate isolate) override;
@@ -88,6 +91,9 @@ class Engine : public UIDelegate,
   void RunFromSnapshotStream(const std::string& name,
                              mojo::ScopedDataPipeConsumerHandle snapshot);
 
+  void StopAnimator();
+  void StartAnimatorIfPossible();
+
   Config config_;
   scoped_ptr<Animator> animator_;
 
@@ -99,6 +105,9 @@ class Engine : public UIDelegate,
   blink::SkyDisplayMetrics display_metrics_;
   mojo::Binding<SkyEngine> binding_;
 
+  bool activity_running_;
+  bool have_surface_;
+
   base::WeakPtrFactory<Engine> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Engine);