Add --use-ime-service command line flag to enable InputMethodMus in --mus.

ash/mus/ash_window_tree_host_mus.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/mus/ash_window_tree_host_mus.h"
 
 #include "ash/host/root_window_transformer.h"
 #include "ash/host/transformer_helper.h"
 #include "ash/ime/input_method_event_handler.h"
 #include "base/memory/ptr_util.h"
@@ skipping 70 matching lines @@
   return transformer_helper_->GetInverseTransform();
 }
 
 void AshWindowTreeHostMus::UpdateRootWindowSizeInPixels(
     const gfx::Size& host_size_in_pixels) {
   transformer_helper_->UpdateWindowSize(host_size_in_pixels);
 }
 
 ui::EventDispatchDetails AshWindowTreeHostMus::DispatchKeyEventPostIME(
     ui::KeyEvent* event) {
-  input_method_handler()->SetPostIME(true);
+  // input_method_handler() can be null when using IME service with --mus.
+  if (input_method_handler())
+    input_method_handler()->SetPostIME(true);
   ui::EventDispatchDetails details = SendEventToSink(event);
-  if (!details.dispatcher_destroyed)
+  if (input_method_handler() && !details.dispatcher_destroyed)
     input_method_handler()->SetPostIME(false);
   return details;
 }
 
 }  // namespace ash