Teach SkyShell.apk to stop posting frames when not visible

sky/services/engine/sky_engine.mojom

 // 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.
 
 module sky;
 
 import "sky/services/engine/input_event.mojom";
 
 struct ViewportMetrics {
   int32 physical_width;
   int32 physical_height;
   float device_pixel_ratio = 1.0;
   double padding_top;
   double padding_right;
   double padding_bottom;
   double padding_left;
 };
 
 interface SkyEngine {
+  OnActivityPaused();
+  OnActivityResumed();
+
   OnViewportMetricsChanged(ViewportMetrics metrics);
   OnInputEvent(InputEvent event);
 
   RunFromNetwork(string url);
   RunFromFile(string main, string package_root);
   RunFromSnapshot(string path);
   RunFromBundle(string path);
 };