WTHDW: Avoid un-necessary local reference to list of open windows.

In SetActiveWindow, we have a local reference for the list of open windows.
How ever, when inserting window handle we create a new reference. This is
un-necessary and the patch fixes it.

impl/window_tree_host_delegate_wayland.cc

@@ -87,7 +87,7 @@ void WindowTreeHostDelegateWayland::SetActiveWindow(
   unsigned window_handle = current_active_window_->window_;
   if (windows.front() != window_handle) {
     windows.remove(window_handle);
-    windows.insert(open_windows().begin(), window_handle);
+    windows.insert(windows.begin(), window_handle);
   }
 
   current_active_window_->Activate();