Make sure WindowObservers are removed from window before destruction

ash/drag_drop/drag_drop_controller.cc

Index: ash/drag_drop/drag_drop_controller.cc
diff --git a/ash/drag_drop/drag_drop_controller.cc b/ash/drag_drop/drag_drop_controller.cc
index 07ab4f39337690f7340edd4d2ce3f2ee6fe5c9cd..1d5d636347eb68d768b6b593e833be773ef39417 100644
--- a/ash/drag_drop/drag_drop_controller.cc
+++ b/ash/drag_drop/drag_drop_controller.cc
@@ -461,14 +461,10 @@ void DragDropController::OnGestureEvent(ui::GestureEvent* event) {
 }
 
 void DragDropController::OnWindowDestroyed(aura::Window* window) {
-  if (drag_window_ == window) {
-    drag_window_->RemoveObserver(this);
+  if (drag_window_ == window)
     drag_window_ = NULL;
-  }
-  if (drag_source_window_ == window) {
-    drag_source_window_->RemoveObserver(this);
+  if (drag_source_window_ == window)
     drag_source_window_ = NULL;
-  }
 }
 
 ////////////////////////////////////////////////////////////////////////////////