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

ash/display/window_tree_host_manager.cc

 // Copyright (c) 2012 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/display/window_tree_host_manager.h"
 
 #include <algorithm>
 #include <cmath>
 #include <map>
 #include <memory>
@@ skipping 808 matching lines @@
   params_with_bounds.display_id = display.id();
   params_with_bounds.device_scale_factor = display.device_scale_factor();
   params_with_bounds.ui_scale_factor = display_info.configured_ui_scale();
   // The AshWindowTreeHost ends up owned by the RootWindowControllers created
   // by this class.
   AshWindowTreeHost* ash_host =
       AshWindowTreeHost::Create(params_with_bounds).release();
   aura::WindowTreeHost* host = ash_host->AsWindowTreeHost();
   // In mash input method is hosted by the browser process, so we don't need to
   // create and set it here.
-  if (Shell::GetAshConfig() != Config::MASH) {
-    DCHECK(!host->has_input_method());

[sky, 2017/05/05 14:22:36]

It would be nice to keep this DCHECK. Otherwise if we accidentally install an
InputMethod we won't know.

[Hadi, 2017/05/05 14:51:57]

Done. Also refactored the condition to Shell::UseIMEService() to not repeat it
in 3 places.

+  if (Shell::GetAshConfig() != Config::MASH && !host->has_input_method()) {
     if (!input_method_) {  // Singleton input method instance for Ash.
       input_method_ = ui::CreateInputMethod(this, host->GetAcceleratedWidget());
       // Makes sure the input method is focused by default when created, because
       // Ash uses singleton InputMethod and it won't call OnFocus/OnBlur when
       // the active window changed.
       input_method_->OnFocus();
       input_method_event_handler_.reset(
           new InputMethodEventHandler(input_method_.get()));
     }
     host->SetSharedInputMethod(input_method_.get());
@@ skipping 14 matching lines @@
 
   window_tree_hosts_[display.id()] = ash_host;
   SetDisplayPropertiesOnHost(ash_host, display);
 
   if (switches::ConstrainPointerToRoot())
     ash_host->ConfineCursorToRootWindow();
   return ash_host;
 }
 
 }  // namespace ash