Centralizes more shared code between ash and mash

ash/mus/test/wm_test_helper.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 ASH_MUS_TEST_WM_TEST_HELPER_H_
 #define ASH_MUS_TEST_WM_TEST_HELPER_H_
 
 #include <memory>
 
 #include "ash/mus/window_manager_application.h"
@@ skipping 33 matching lines @@
   }
 
   // Returns the RootWindowControllers ordered by display id (which we assume
   // correlates with creation order).
   std::vector<RootWindowController*> GetRootsOrderedByDisplayId();
 
   void UpdateDisplay(const std::string& display_spec);
 
  private:
   // Creates a new RootWindowController given |display_spec|, which is the
-  // configuration of the display.
+  // configuration of the display. On entry |next_x| is the x-coordinate to
+  // place the display at, on exit |next_x| is set to the x-coordinate to place
+  // the next display at.
   RootWindowController* CreateRootWindowController(
-      const std::string& display_spec);
+      const std::string& display_spec,
+      int* next_x);
 
-  // Updates the display of an existing RootWindowController.
+  // Updates the display of an existing RootWindowController. See
+  // CreateRootWindowController() for details on |next_x|.
   void UpdateDisplay(RootWindowController* root_window_controller,
-                     const std::string& display_spec);
+                     const std::string& display_spec,
+                     int* next_x);
 
   // Destroys a RootWindowController.
   void DestroyRootWindowController(
       RootWindowController* root_window_controller);
 
   std::unique_ptr<base::MessageLoop> message_loop_;
   ui::TestWindowTreeClientSetup window_tree_client_setup_;
   std::unique_ptr<WindowManagerApplication> window_manager_app_;
   std::unique_ptr<ui::WindowTreeClientPrivate> window_tree_client_private_;
 
   // Id for the next Display created by CreateRootWindowController().
   int64_t next_display_id_ = 1;
 
   std::unique_ptr<base::SequencedWorkerPoolOwner> blocking_pool_owner_;
 
   DISALLOW_COPY_AND_ASSIGN(WmTestHelper);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_TEST_WM_TEST_HELPER_H_