Adds support for multiple displays to WmTestBase

ash/mus/test/wm_test_base.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_BASE_H_
 #define ASH_MUS_TEST_WM_TEST_BASE_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 51 matching lines @@
   // Creates a window parented to |parent|. The returned window is visible.
   ui::Window* CreateChildTestWindow(ui::Window* parent,
                                     const gfx::Rect& bounds);
 
  protected:
   // testing::Test:
   void SetUp() override;
   void TearDown() override;
 
  private:
-  // Returns the RootWindowControllers ordered by display id (which we assume
-  // correlates with creation order).
-  std::vector<RootWindowController*> GetRootsOrderedByDisplayId();
-
   bool setup_called_ = false;
   bool teardown_called_ = false;
   std::unique_ptr<WmTestHelper> test_helper_;
 
   DISALLOW_COPY_AND_ASSIGN(WmTestBase);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_TEST_WM_TEST_BASE_H_