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

ash/display/display_util.h

 // Copyright 2014 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_UTIL_H_
 #define ASH_DISPLAY_DISPLAY_UTIL_H_
 
 #include <stdint.h>
 
 #include <set>
@@ skipping 71 matching lines @@
 void MoveCursorTo(AshWindowTreeHost* ash_host,
                   const gfx::Point& point_in_screen,
                   bool update_last_location_now);
 
 // Returns the index in the displays whose bounds contains |point_in_screen|.
 // Returns -1 if no such display exist.
 ASH_EXPORT int FindDisplayIndexContainingPoint(
     const std::vector<gfx::Display>& displays,
     const gfx::Point& point_in_screen);
 
-// Creates the DisplayIdPair where ids are sorted using |CompareDisplayIds|
+// Creates the DisplayIdList where ids are sorted using |CompareDisplayIds|
 // below.
-ASH_EXPORT DisplayIdPair CreateDisplayIdPair(int64_t id1, int64_t id2);
+ASH_EXPORT DisplayIdList CreateDisplayIdList(int64_t id1, int64_t id2);
+
+ASH_EXPORT std::string DisplayIdListToString(const DisplayIdList& list);
 
 // Returns true if one of following conditinos is met.
 // 1) id1 is internal.
 // 2) output index of id1 < output index of id2 and id2 isn't internal.
 ASH_EXPORT bool CompareDisplayIds(int64_t id1, int64_t id2);
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_UTIL_H_