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

ash/display/display_change_observer_chromeos.cc

Index: ash/display/display_change_observer_chromeos.cc
diff --git a/ash/display/display_change_observer_chromeos.cc b/ash/display/display_change_observer_chromeos.cc
index 3f820a90ac23483f4c906d655a572b86bf7149ef..97a76e0bc979cdaeb3747aeda2a585483baf1809 100644
--- a/ash/display/display_change_observer_chromeos.cc
+++ b/ash/display/display_change_observer_chromeos.cc
@@ -156,10 +156,12 @@ ui::MultipleDisplayState DisplayChangeObserver::GetStateForDisplayIds(
   UpdateInternalDisplayId(display_states);
   if (display_states.size() != 2)
     return ui::MULTIPLE_DISPLAY_STATE_DUAL_EXTENDED;
-  DisplayIdPair pair = CreateDisplayIdPair(display_states[0]->display_id(),
+  DisplayIdList list = CreateDisplayIdList(display_states[0]->display_id(),
                                            display_states[1]->display_id());
-  DisplayLayout layout = Shell::GetInstance()->display_manager()->
-      layout_store()->GetRegisteredDisplayLayout(pair);
+  DisplayLayout layout = Shell::GetInstance()
+                             ->display_manager()
+                             ->layout_store()
+                             ->GetRegisteredDisplayLayout(list);
   return layout.mirrored ? ui::MULTIPLE_DISPLAY_STATE_DUAL_MIRROR :
                            ui::MULTIPLE_DISPLAY_STATE_DUAL_EXTENDED;
 }