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

ui/wm/core/window_modality_controller.h

 // Copyright (c) 2012 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_WM_CORE_WINDOW_MODALITY_CONTROLLER_H_
 #define UI_WM_CORE_WINDOW_MODALITY_CONTROLLER_H_
 
 #include <vector>
 
 #include "base/compiler_specific.h"
@@ skipping 31 matching lines @@
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnTouchEvent(ui::TouchEvent* event) override;
 
   // Overridden from aura::EnvObserver:
   void OnWindowInitialized(aura::Window* window) override;
 
   // Overridden from aura::WindowObserver:
   void OnWindowPropertyChanged(aura::Window* window,
                                const void* key,
                                intptr_t old) override;
-  void OnWindowVisibilityChanged(aura::Window* window, bool visible) override;
+  void OnWindowVisibilityChanged(aura::Window* window, bool visible,
+                                 bool content_visible) override;
   void OnWindowDestroyed(aura::Window* window) override;
 
  private:
   // Processes a mouse/touch event, and returns true if the event should be
   // consumed.
   bool ProcessLocatedEvent(aura::Window* target,
                            ui::LocatedEvent* event);
 
   std::vector<aura::Window*> windows_;
 
   ui::EventTarget* event_target_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowModalityController);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_WINDOW_MODALITY_CONTROLLER_H_