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

ash/ash_switches.cc

Index: ash/ash_switches.cc
diff --git a/ash/ash_switches.cc b/ash/ash_switches.cc
index 0f55bdfefbebcc901c1dbc37f1bd696d60dff55c..0e75f12ae4295faf7b730d93f1f10a9673e68f39 100644
--- a/ash/ash_switches.cc
+++ b/ash/ash_switches.cc
@@ -107,6 +107,10 @@ const char kAshTouchHud[] = "ash-touch-hud";
 // instead of displaying an interactive animation.
 const char kAuraLegacyPowerButton[] = "aura-legacy-power-button";
 
+// By default we use classic IME (i.e. InputMethodChromeOS) in kMus. This flag
+// enables the IME service (i.e. InputMethodMus) instead.
+const char kUseIMEService[] = "use-ime-service";
+
 // Constrains the pointer movement within a root window on desktop.
 bool ConstrainPointerToRoot() {
   const char kAshConstrainPointerToRoot[] = "ash-constrain-pointer-to-root";