Allow dynamic enabling/disabling of unified desktop

ash/display/display_layout.h

 // Copyright (c) 2013 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_DISPLAY_DISPLAY_LAYOUT_H_
 #define ASH_DISPLAY_DISPLAY_LAYOUT_H_
 
 #include <map>
 #include <string>
 
@@ skipping 48 matching lines @@
   // True if displays are mirrored.
   bool mirrored;
 
   // True if multi displays should default to unified mode.
   bool default_unified;
 
   // The id of the display used as a primary display.
   int64 primary_id;
 
   // Returns string representation of the layout for debugging/testing.
+  // This includes "unified" only if the unified desktop feature is enabled.
   std::string ToString() const;
 };
 
 }  // namespace ash
 
 #endif