Converts WindowTreeClientTest to be in terms of aura

ui/aura/test/mus/window_tree_client_private.cc

Index: ui/aura/test/mus/window_tree_client_private.cc
diff --git a/ui/aura/test/mus/window_tree_client_private.cc b/ui/aura/test/mus/window_tree_client_private.cc
index 9157270ace059bc5a4b90888f9f25b564588005f..92c421268e579545a528dd72f864574bf3a46966 100644
--- a/ui/aura/test/mus/window_tree_client_private.cc
+++ b/ui/aura/test/mus/window_tree_client_private.cc
@@ -79,4 +79,9 @@ bool WindowTreeClientPrivate::HasPointerWatcher() {
   return tree_client_impl_->has_pointer_watcher_;
 }
 
+Window* WindowTreeClientPrivate::GetWindowByServerId(Id id) {
+  WindowMus* window = tree_client_impl_->GetWindowByServerId(id);
+  return window ? window->GetWindow() : nullptr;
+}
+
 }  // namespace aura