Fixes shutdown race in mash

ash/mus/test/wm_test_helper.cc

Index: ash/mus/test/wm_test_helper.cc
diff --git a/ash/mus/test/wm_test_helper.cc b/ash/mus/test/wm_test_helper.cc
index 0fbf42661a7ab1dfefa2673d4edd6448c9e39c27..a14b0381426b0ef44714edf8894c0f8aa40bdb03 100644
--- a/ash/mus/test/wm_test_helper.cc
+++ b/ash/mus/test/wm_test_helper.cc
@@ -24,8 +24,7 @@ WmTestHelper::~WmTestHelper() {}
 
 void WmTestHelper::Init() {
   message_loop_.reset(new base::MessageLoopForUI());
-  window_manager_app_.window_manager_.reset(
-      new WindowManager(&window_manager_app_, nullptr));
+  window_manager_app_.window_manager_.reset(new WindowManager(nullptr));
   screen_ = new WmTestScreen;
   window_manager_app_.window_manager_->screen_.reset(screen_);