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

ash/display/window_tree_host_manager.cc

Index: ash/display/window_tree_host_manager.cc
diff --git a/ash/display/window_tree_host_manager.cc b/ash/display/window_tree_host_manager.cc
index eedc108dcbabc3e860e736e2c7f250a978191f53..d15975c462e89bc6977b889d65c50352871cb8a2 100644
--- a/ash/display/window_tree_host_manager.cc
+++ b/ash/display/window_tree_host_manager.cc
@@ -826,8 +826,7 @@ AshWindowTreeHost* WindowTreeHostManager::AddWindowTreeHostForDisplay(
   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