Adds View::visible() and IsDrawn

mojo/services/public/cpp/view_manager/view_observer.h

Index: mojo/services/public/cpp/view_manager/view_observer.h
diff --git a/mojo/services/public/cpp/view_manager/view_observer.h b/mojo/services/public/cpp/view_manager/view_observer.h
index 2be9dbcdfe3c325a50dee5f6e205f4dd1e880adb..1a2ab7e2283b176a43e499daaa16b258a6d54a59 100644
--- a/mojo/services/public/cpp/view_manager/view_observer.h
+++ b/mojo/services/public/cpp/view_manager/view_observer.h
@@ -64,6 +64,14 @@ class ViewObserver {
 
   virtual void OnViewInputEvent(View* view, const EventPtr& event) {}
 
+  virtual void OnViewVisibilityChanging(View* view) {}
+  virtual void OnViewVisibilityChanged(View* view) {}
+
+  // Sent when the drawn state changes. This is only sent for the root nodes
+  // when embedded.
+  virtual void OnViewDrawnChanging(View* view) {}
+  virtual void OnViewDrawnChanged(View* view) {}
+
  protected:
   virtual ~ViewObserver() {}
 };