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

ash/display/display_layout.h

Index: ash/display/display_layout.h
diff --git a/ash/display/display_layout.h b/ash/display/display_layout.h
index 23e5779f589bda5a8a787b53cda55c928a69f6c3..3f4c56aecd0c9a8e39663e5226d386db8c24937e 100644
--- a/ash/display/display_layout.h
+++ b/ash/display/display_layout.h
@@ -7,8 +7,8 @@
 
 #include <stdint.h>
 
-#include <map>
 #include <string>
+#include <vector>
 
 #include "ash/ash_export.h"
 #include "base/compiler_specific.h"
@@ -20,7 +20,7 @@ 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.