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

ui/views/widget/desktop_aura/desktop_native_widget_aura.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_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_NATIVE_WIDGET_AURA_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_NATIVE_WIDGET_AURA_H_
 
 #include "base/memory/weak_ptr.h"
 #include "ui/aura/client/focus_change_observer.h"
 #include "ui/aura/window_delegate.h"
@@ skipping 216 matching lines @@
   void OnDragEntered(const ui::DropTargetEvent& event) override;
   int OnDragUpdated(const ui::DropTargetEvent& event) override;
   void OnDragExited() override;
   int OnPerformDrop(const ui::DropTargetEvent& event) override;
 
   // Overridden from aura::WindowTreeHostObserver:
   void OnHostCloseRequested(const aura::WindowTreeHost* host) override;
   void OnHostResized(const aura::WindowTreeHost* host) override;
   void OnHostMoved(const aura::WindowTreeHost* host,
                    const gfx::Point& new_origin) override;
+  void OnHostSetWindowVisibility(const aura::WindowTreeHost* host,
+                                 bool visible) override {}
 
  private:
   friend class FocusManagerEventHandler;
   friend class RootWindowDestructionObserver;
 
   // Installs the input method filter.
   void InstallInputMethodEventFilter();
 
   // To save a clear on platforms where the window is never transparent, the
   // window is only set as transparent when the glass frame is in use.
@@ skipping 68 matching lines @@
   // The following factory is used for calls to close the NativeWidgetAura
   // instance.
   base::WeakPtrFactory<DesktopNativeWidgetAura> close_widget_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopNativeWidgetAura);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_NATIVE_WIDGET_AURA_H_