Changes how window manager obtains WindowTree from mus

ash/mus/test/wm_test_base.cc

Index: ash/mus/test/wm_test_base.cc
diff --git a/ash/mus/test/wm_test_base.cc b/ash/mus/test/wm_test_base.cc
index f99703ce86b169c2ee8c7de08c8ced0db613e0d9..0811793ee3c5998795d91b08b902b8269c300904 100644
--- a/ash/mus/test/wm_test_base.cc
+++ b/ash/mus/test/wm_test_base.cc
@@ -200,8 +200,9 @@ void WmTestBase::TearDown() {
 }
 
 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);