IME for Mus: Send ack for key events after IME driver processes the event.

ui/views/mus/native_widget_mus.h

Index: ui/views/mus/native_widget_mus.h
diff --git a/ui/views/mus/native_widget_mus.h b/ui/views/mus/native_widget_mus.h
index 257038bb1890169ad1ba4106e3fb2698148abfc4..3b932987d8ed447840726cdb7d998f613428635f 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -59,6 +59,7 @@ class TooltipController;
 }
 class DropHelper;
 class DropTargetMus;
+class InputMethodMus;
 class TooltipManagerAura;
 class WidgetDelegate;
 
@@ -301,6 +302,8 @@ class VIEWS_MUS_EXPORT NativeWidgetMus
   std::unique_ptr<corewm::TooltipController> tooltip_controller_;
   std::unique_ptr<TooltipManagerAura> tooltip_manager_;
 
+  std::unique_ptr<InputMethodMus> input_method_;
+
   base::WeakPtrFactory<NativeWidgetMus> close_widget_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeWidgetMus);