Make Page Visibility API work when the browser window is visible or not

ui/aura/window_tree_host_observer.h

 // Copyright 2014 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 UI_AURA_WINDOW_TREE_HOST_OBSERVER_H_
 #define UI_AURA_WINDOW_TREE_HOST_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace gfx {
 class Point;
 }
 
 namespace aura {
 class Window;
 class WindowTreeHost;
 
 class AURA_EXPORT WindowTreeHostObserver {
  public:
   // Called when the host's client size has changed.
   virtual void OnHostResized(const WindowTreeHost* host) {}
 
   // Called when the host is moved on screen.
   virtual void OnHostMoved(const WindowTreeHost* host,
                            const gfx::Point& new_origin) {}
 
   // Called when the native window system sends the host request to close.
   virtual void OnHostCloseRequested(const WindowTreeHost* host) {}
 
+  // Called when the host becomes visible or hidden.
+  virtual void OnHostSetWindowVisibility(const WindowTreeHost* host,
+                                         bool visible) {}
+
  protected:
   virtual ~WindowTreeHostObserver() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_TREE_HOST_OBSERVER_H_