Changes how window manager obtains WindowTree from mus

ash/mus/test/wm_test_base.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/mus/test/wm_test_base.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "ash/mus/root_window_controller.h"
@@ skipping 182 matching lines @@
   test_helper_.reset(new WmTestHelper);
   test_helper_->Init();
 }
 
 void WmTestBase::TearDown() {
   teardown_called_ = true;
   test_helper_.reset();
 }
 
 std::vector<RootWindowController*> WmTestBase::GetRootsOrderedByDisplayId() {
-  std::set<RootWindowController*> roots =
-      test_helper_->window_manager_app()->GetRootControllers();
+  std::set<RootWindowController*> roots = test_helper_->window_manager_app()
+                                              ->window_manager()
+                                              ->GetRootWindowControllers();
   std::vector<RootWindowController*> ordered_roots;
   ordered_roots.insert(ordered_roots.begin(), roots.begin(), roots.end());
   std::sort(ordered_roots.begin(), ordered_roots.end(), &CompareByDisplayId);
   return ordered_roots;
 }
 
 }  // namespace mus
 }  // namespace ash