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

ui/aura/window_event_dispatcher.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_EVENT_DISPATCHER_H_
 #define UI_AURA_WINDOW_EVENT_DISPATCHER_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 181 matching lines @@
   void DispatchGestureEvent(ui::GestureEvent* event) override;
   void DispatchCancelTouchEvent(ui::TouchEvent* event) override;
 
   // Overridden from WindowObserver:
   void OnWindowDestroying(Window* window) override;
   void OnWindowDestroyed(Window* window) override;
   void OnWindowAddedToRootWindow(Window* window) override;
   void OnWindowRemovingFromRootWindow(Window* window,
                                       Window* new_root) override;
   void OnWindowVisibilityChanging(Window* window, bool visible) override;
-  void OnWindowVisibilityChanged(Window* window, bool visible) override;
+  void OnWindowVisibilityChanged(Window* window, bool visible,
+                                 bool content_visible) override;
   void OnWindowBoundsChanged(Window* window,
                              const gfx::Rect& old_bounds,
                              const gfx::Rect& new_bounds) override;
   void OnWindowTransforming(Window* window) override;
   void OnWindowTransformed(Window* window) override;
 
   // Overridden from EnvObserver:
   void OnWindowInitialized(Window* window) override;
 
   // We hold and aggregate mouse drags and touch moves as a way of throttling
@@ skipping 52 matching lines @@
 
   // Used to schedule DispatchHeldEvents() when |move_hold_count_| goes to 0.
   base::WeakPtrFactory<WindowEventDispatcher> held_event_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowEventDispatcher);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_EVENT_DISPATCHER_H_