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

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 <stdint.h>
 
-#include <map>
 #include <string>
+#include <vector>
 
 #include "ash/ash_export.h"
 #include "base/compiler_specific.h"
 
 namespace base {
 class Value;
 template <typename T> class JSONValueConverter;
 }
 
 namespace ash {
 
-typedef std::pair<int64_t, int64_t> DisplayIdPair;
+typedef std::vector<int64_t> DisplayIdList;
 
 struct ASH_EXPORT DisplayLayout {
   // Layout options where the secondary display should be positioned.
   enum Position {
     TOP,
     RIGHT,
     BOTTOM,
     LEFT
   };
 
@@ skipping 33 matching lines @@
   int64_t 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