Converts WindowTreeClientTest to be in terms of aura

ui/aura/test/mus/window_tree_client_private.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/test/mus/window_tree_client_private.h"
 
 #include "ui/aura/mus/window_port_mus.h"
 #include "ui/aura/mus/window_tree_client.h"
 #include "ui/aura/window.h"
 #include "ui/display/display.h"
@@ skipping 61 matching lines @@
     ui::mojom::WindowTree* window_tree,
     ClientSpecificId client_id) {
   tree_client_impl_->WindowTreeConnectionEstablished(window_tree);
   tree_client_impl_->client_id_ = client_id;
 }
 
 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