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

ash/mus/ash_window_tree_host_mus.cc

Index: ash/mus/ash_window_tree_host_mus.cc
diff --git a/ash/mus/ash_window_tree_host_mus.cc b/ash/mus/ash_window_tree_host_mus.cc
index 371285523a74bad2c571043e618e09f36739f9ae..7c10818a2cb53502302e5b63d78e7964aa72d2ac 100644
--- a/ash/mus/ash_window_tree_host_mus.cc
+++ b/ash/mus/ash_window_tree_host_mus.cc
@@ -88,9 +88,11 @@ void AshWindowTreeHostMus::UpdateRootWindowSizeInPixels(
 
 ui::EventDispatchDetails AshWindowTreeHostMus::DispatchKeyEventPostIME(
     ui::KeyEvent* event) {
-  input_method_handler()->SetPostIME(true);
+  // input_method_handler() can be null when using IME service with --mus.
+  if (input_method_handler())
+    input_method_handler()->SetPostIME(true);
   ui::EventDispatchDetails details = SendEventToSink(event);
-  if (!details.dispatcher_destroyed)
+  if (input_method_handler() && !details.dispatcher_destroyed)
     input_method_handler()->SetPostIME(false);
   return details;
 }