Use list instead of pair to represent the set of displays.

ash/display/display_manager.h

 // 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.
 
 #ifndef ASH_DISPLAY_DISPLAY_MANAGER_H_
 #define ASH_DISPLAY_DISPLAY_MANAGER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 117 matching lines @@
   // Initialize default display.
   void InitDefaultDisplay();
 
   // Initializes font related params that depends on display
   // configuration.
   void RefreshFontParams();
 
   // Returns the display layout used for current displays.
   DisplayLayout GetCurrentDisplayLayout();
 
-  // Returns the current display pair.
-  DisplayIdPair GetCurrentDisplayIdPair() const;
+  // Returns the current display list.
+  DisplayIdList GetCurrentDisplayIdList() const;
 
   // Sets the layout for the current display pair. The |layout| specifies
   // the locaion of the secondary display relative to the primary.
   void SetLayoutForCurrentDisplays(
       const DisplayLayout& layout_relative_to_primary);
 
   // Returns display for given |id|;
   const gfx::Display& GetDisplayForId(int64_t id) const;
 
   // Finds the display that contains |point| in screeen coordinates.
@@ skipping 298 matching lines @@
   bool unified_desktop_enabled_;
 
   base::WeakPtrFactory<DisplayManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayManager);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_MANAGER_H_