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..437aa136586232f431bc66495ad7128e2aea5fc6 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"
@@ -309,6 +310,14 @@ Config Shell::GetAshConfig() {
   return Get()->shell_port_->GetAshConfig();
 }
 
+// static
+bool Shell::UseIMEService() {
+  return Shell::GetAshConfig() == Config::MASH ||
+         (Shell::GetAshConfig() == Config::MUS &&
+          base::CommandLine::ForCurrentProcess()->HasSwitch(
+              switches::kUseIMEService));
+}
+
 views::NonClientFrameView* Shell::CreateDefaultNonClientFrameView(
     views::Widget* widget) {
   // Use translucent-style window frames for dialogs.
@@ -935,8 +944,9 @@ 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) {
-    // Not applicable to mash as events are already routed to InputMethod first.
+  if (!UseIMEService()) {
+    // Not applicable when using IME service as events are already routed to
+    // InputMethod first.
     AddPreTargetHandler(
         window_tree_host_manager_->input_method_event_handler());
   }