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

ash/display/display_util.cc

 // 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.
 
 #include "ash/display/display_util.h"
 
 #include <algorithm>
 
 #include "ash/display/display_info.h"
 #include "ash/display/display_manager.h"
 #include "ash/host/ash_window_tree_host.h"
 #include "ash/shell.h"
+#include "base/strings/string_number_conversions.h"
 #include "ui/aura/env.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/gfx/display.h"
 #include "ui/gfx/geometry/point.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size_conversions.h"
 #include "ui/wm/core/coordinate_conversion.h"
 
 #if defined(OS_CHROMEOS)
 #include "base/sys_info.h"
@@ skipping 318 matching lines @@
 
 int FindDisplayIndexContainingPoint(const std::vector<gfx::Display>& displays,
                                     const gfx::Point& point_in_screen) {
   auto iter = std::find_if(displays.begin(), displays.end(),
                            [point_in_screen](const gfx::Display& display) {
                              return display.bounds().Contains(point_in_screen);
                            });
   return iter == displays.end() ? -1 : (iter - displays.begin());
 }
 
-DisplayIdPair CreateDisplayIdPair(int64_t id1, int64_t id2) {
-  return CompareDisplayIds(id1, id2) ? std::make_pair(id1, id2)
-                                     : std::make_pair(id2, id1);
+DisplayIdList CreateDisplayIdList(int64_t id1, int64_t id2) {
+  std::vector<int64_t> ids;
+
+  ids.push_back(id1);
+  ids.push_back(id2);
+  std::sort(ids.begin(), ids.end(),
+            [](int64_t a, int64_t b) { return CompareDisplayIds(a, b); });
+  return ids;
+}
+
+std::string DisplayIdListToString(const ash::DisplayIdList& list) {
+  return base::Int64ToString(list[0]) + "," + base::Int64ToString(list[1]);
 }
 
 bool CompareDisplayIds(int64_t id1, int64_t id2) {
   DCHECK_NE(id1, id2);
   // Output index is stored in the first 8 bits. See GetDisplayIdFromEDID
   // in edid_parser.cc.
   int index_1 = id1 & 0xFF;
   int index_2 = id2 & 0xFF;
   DCHECK_NE(index_1, index_2) << id1 << " and " << id2;
   return gfx::Display::IsInternalDisplayId(id1) ||
          (index_1 < index_2 && !gfx::Display::IsInternalDisplayId(id2));
 }
 
 }  // namespace ash