Converts WindowTreeClientTest to be in terms of aura

ui/aura/test/mus/window_tree_client_private.h

 // 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.
 
 #ifndef UI_AURA_TEST_MUS_WINDOW_TREE_CLIENT_PRIVATE_H_
 #define UI_AURA_TEST_MUS_WINDOW_TREE_CLIENT_PRIVATE_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 39 matching lines @@
   void CallOnWindowInputEvent(Window* window, std::unique_ptr<ui::Event> event);
 
   void CallOnCaptureChanged(Window* new_capture, Window* old_capture);
 
   // Sets the WindowTree and client id.
   void SetTreeAndClientId(ui::mojom::WindowTree* window_tree,
                           ClientSpecificId client_id);
 
   bool HasPointerWatcher();
 
+  Window* GetWindowByServerId(Id id);
+
  private:
   WindowTreeClient* tree_client_impl_;
   uint16_t next_window_id_ = 1u;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClientPrivate);
 };
 
 }  // namespace ui
 
 #endif  // UI_AURA_TEST_MUS_WINDOW_TREE_CLIENT_PRIVATE_H_