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

ui/aura/window_observer.h

Index: ui/aura/window_observer.h
diff --git a/ui/aura/window_observer.h b/ui/aura/window_observer.h
index f4e7237f990d23fc2d740f4055554dcbdecc17fa..d20a071d96efe833cd7c955da7d5de9dc778303d 100644
--- a/ui/aura/window_observer.h
+++ b/ui/aura/window_observer.h
@@ -70,7 +70,8 @@ class AURA_EXPORT WindowObserver {
   // value supplied to SetVisible(). If |visible| is true, window->IsVisible()
   // may still return false. See description in Window::IsVisible() for details.
   virtual void OnWindowVisibilityChanging(Window* window, bool visible) {}
-  virtual void OnWindowVisibilityChanged(Window* window, bool visible) {}
+  virtual void OnWindowVisibilityChanged(Window* window, bool visible,
+                                         bool content_visible) {}
 
   // Invoked when SetBounds() is invoked on |window|. |old_bounds| and
   // |new_bounds| are in parent coordinates.