Separate ash::test::DisplayManagerTestApi from ash

ash/display/display_manager.cc

 // Copyright (c) 2012 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 "ash/display/display_manager.h"
 
 #include <algorithm>
 #include <cmath>
 #include <limits>
 #include <map>
@@ skipping 1415 matching lines @@
 }
 
 void DisplayManager::AddObserver(display::DisplayObserver* observer) {
   observers_.AddObserver(observer);
 }
 
 void DisplayManager::RemoveObserver(display::DisplayObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
+const display::Display& DisplayManager::GetSecondaryDisplay() const {
+  CHECK_LE(2U, GetNumDisplays());
+  return GetDisplayAt(0).id() ==
+                 display::Screen::GetScreen()->GetPrimaryDisplay().id()
+             ? GetDisplayAt(1)
+             : GetDisplayAt(0);
+}
+
 }  // namespace ash