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

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 4ba0a582088f1b42b8aaae87dea89723fe347fd0..bb3241f7b3d69475db51a2eb155b9cb3ace5250c 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -16,6 +16,7 @@
 #include "ash/accessibility_delegate.h"
 #include "ash/app_list/app_list_delegate_impl.h"
 #include "ash/ash_constants.h"
+#include "ash/ash_switches.h"
 #include "ash/aura/shell_port_classic.h"
 #include "ash/autoclick/autoclick_controller.h"
 #include "ash/cast_config_controller.h"
@@ -935,7 +936,10 @@ void Shell::Init(const ShellInitParams& init_params) {
   accelerator_controller_ = shell_port_->CreateAcceleratorController();
   maximize_mode_controller_ = base::MakeUnique<MaximizeModeController>();
 
-  if (config == Config::CLASSIC || config == Config::MUS) {
+  if (config == Config::CLASSIC ||
+      (config == Config::MUS &&
+       !base::CommandLine::ForCurrentProcess()->HasSwitch(
+           switches::kUseServicifiedIME))) {
     // Not applicable to mash as events are already routed to InputMethod first.
     AddPreTargetHandler(
         window_tree_host_manager_->input_method_event_handler());