Converts test_wm to use aura

ui/aura/test/aura_test_base.cc

Index: ui/aura/test/aura_test_base.cc
diff --git a/ui/aura/test/aura_test_base.cc b/ui/aura/test/aura_test_base.cc
index 75fa47eb0fcc40dbbec3db68ff7cd4867050b46e..4ed27d2bba58f75b61cbff627017dfa45d56b7ba 100644
--- a/ui/aura/test/aura_test_base.cc
+++ b/ui/aura/test/aura_test_base.cc
@@ -188,7 +188,10 @@ void AuraTestBase::OnWmNewDisplay(
   window_tree_host_mus_ = std::move(window_tree_host);
 }
 
-void AuraTestBase::OnWmDisplayRemoved(Window* window) {}
+void AuraTestBase::OnWmDisplayRemoved(WindowTreeHostMus* window_tree_host) {
+  if (window_tree_host_mus_.get() == window_tree_host)
+    window_tree_host_mus_.reset();
+}
 
 void AuraTestBase::OnWmDisplayModified(const display::Display& display) {}