mus: Allow the WM to specify the windows that can have active children.

components/mus/ws/window_manager_client_apptest.cc

Index: components/mus/ws/window_manager_client_apptest.cc
diff --git a/components/mus/ws/window_manager_client_apptest.cc b/components/mus/ws/window_manager_client_apptest.cc
index e38b76347462490fe2b05626ea871bcd786be3e9..74fd5aa5181542e2da734366546596edbc9d9975 100644
--- a/components/mus/ws/window_manager_client_apptest.cc
+++ b/components/mus/ws/window_manager_client_apptest.cc
@@ -644,8 +644,7 @@ TEST_F(WindowServerTest, Focus) {
   }
   {
     // Add an observer on the Window that loses focus, and make sure the
-    // observer
-    // sees the right values.
+    // observer sees the right values.
     FocusChangeObserver observer(window11);
     embedded->GetRoot()->SetFocus();
     ASSERT_TRUE(DoRunLoopWithTimeout());