Unified Desktop: hook up ash to allow unified desktop mode

ash/host/ash_window_tree_host.h

Index: ash/host/ash_window_tree_host.h
diff --git a/ash/host/ash_window_tree_host.h b/ash/host/ash_window_tree_host.h
index 321ee25c499a559c5ae3765ebee89f8a502825f0..fab7afd1f7bbde725f594175eb5ed6a8f45f9cd7 100644
--- a/ash/host/ash_window_tree_host.h
+++ b/ash/host/ash_window_tree_host.h
@@ -55,6 +55,8 @@ class ASH_EXPORT AshWindowTreeHost {
   // Stop listening for events in preparation for shutdown.
   virtual void PrepareForShutdown() {}
 
+  virtual void RegisterMirroringHost(AshWindowTreeHost* mirroring_ash_host) {}
+
  protected:
   // Translates the native mouse location into screen coordinates.
   void TranslateLocatedEvent(ui::LocatedEvent* event);