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

ui/aura/window_tree_host.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_AURA_WINDOW_TREE_HOST_H_
 #define UI_AURA_WINDOW_TREE_HOST_H_
 
 #include <vector>
 
 #include "base/event_types.h"
@@ skipping 138 matching lines @@
   void CreateCompositor(gfx::AcceleratedWidget accelerated_widget);
 
   // Returns the location of the RootWindow on native screen.
   virtual gfx::Point GetLocationOnNativeScreen() const = 0;
 
   void OnHostMoved(const gfx::Point& new_location);
   void OnHostResized(const gfx::Size& new_size);
   void OnHostCloseRequested();
   void OnHostActivated();
   void OnHostLostWindowCapture();
+  void OnHostSetWindowVisibility(bool visible);
 
   // Sets the currently displayed cursor.
   virtual void SetCursorNative(gfx::NativeCursor cursor) = 0;
 
   // Moves the cursor to the specified location relative to the root window.
   virtual void MoveCursorToNative(const gfx::Point& location) = 0;
 
   // kCalled when the cursor visibility has changed.
   virtual void OnCursorVisibilityChangedNative(bool show) = 0;
 
@@ skipping 22 matching lines @@
   gfx::Point last_cursor_request_position_in_host_;
 
   scoped_ptr<ui::ViewProp> prop_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeHost);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_TREE_HOST_H_