window-service: adds test coverage for deleting a top-level locally.

BUG=none
TEST=covered by test

Change-Id: I1402a8f68f1db6870cf317dc0a7e3ae565927949
Reviewed-on: https://chromium-review.googlesource.com/1164530Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581112}

services/ui/ws2/window_tree.cc

@@ -1171,11 +1171,8 @@ void WindowTree::OnWindowDestroyed(aura::Window* window) {
   // destructor.
 
   auto iter = FindClientRootWithRoot(window);
-  if (iter != client_roots_.end()) {
-    // TODO(sky): add test coverage of this. I'm pretty sure this isn't wired
-    // up correctly.
+  if (iter != client_roots_.end())
     DeleteClientRoot(iter->get(), WindowTree::DeleteClientRootReason::kDeleted);
-  }
 
   DCHECK_NE(0u, window_to_client_window_id_map_.count(window));
   const ClientWindowId client_window_id =

services/ui/ws2/window_tree_unittest.cc

@@ -996,6 +996,17 @@ TEST(WindowTreeTest, DeleteTopLevel) {
       setup.window_tree_test_helper()->GetWindowByClientId(top_level_id));
 }
 
+TEST(WindowTreeTest, ExternalDeleteTopLevel) {
+  WindowServiceTestSetup setup;
+  aura::Window* top_level =
+      setup.window_tree_test_helper()->NewTopLevelWindow();
+  setup.changes()->clear();
+  ASSERT_TRUE(top_level);
+  delete top_level;
+  EXPECT_EQ("WindowDeleted window=0,1",
+            SingleChangeToDescription(*setup.changes()));
+}
+
 TEST(WindowTreeTest, ExternalDeleteWindow) {
   WindowServiceTestSetup setup;
   aura::Window* window = setup.window_tree_test_helper()->NewWindow();