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

ui/aura/window_tree_host_observer.h

Index: ui/aura/window_tree_host_observer.h
diff --git a/ui/aura/window_tree_host_observer.h b/ui/aura/window_tree_host_observer.h
index d96e9d3b24dc1230deb2ab5920787bcbff2db98f..82badfb669df607c9ce4146e2676ae4d3494eff5 100644
--- a/ui/aura/window_tree_host_observer.h
+++ b/ui/aura/window_tree_host_observer.h
@@ -27,6 +27,10 @@ class AURA_EXPORT WindowTreeHostObserver {
   // Called when the native window system sends the host request to close.
   virtual void OnHostCloseRequested(const WindowTreeHost* host) {}
 
+  // Called when the host becomes visible or hidden.
+  virtual void OnHostSetWindowVisibility(const WindowTreeHost* host,
+                                         bool visible) {}
+
  protected:
   virtual ~WindowTreeHostObserver() {}
 };