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

ash/ash_switches.h

Index: ash/ash_switches.h
diff --git a/ash/ash_switches.h b/ash/ash_switches.h
index 68ec242b0523fc9c5739ab1b5591600f132010ea..10f5beee87db96a72e2734fdd39ec9d4dfa056c8 100644
--- a/ash/ash_switches.h
+++ b/ash/ash_switches.h
@@ -47,6 +47,7 @@ ASH_EXPORT extern const char kAshShelfColorSchemeDarkMuted[];
 ASH_EXPORT extern const char kAshShelfColorSchemeDarkVibrant[];
 ASH_EXPORT extern const char kAshTouchHud[];
 ASH_EXPORT extern const char kAuraLegacyPowerButton[];
+ASH_EXPORT extern const char kUseIMEService[];
 
 // True if the pointer (cursor) position should be kept inside root windows.
 ASH_EXPORT bool ConstrainPointerToRoot();