Allow switching between extended and unified

ash/display/display_manager.h

Index: ash/display/display_manager.h
diff --git a/ash/display/display_manager.h b/ash/display/display_manager.h
index b6ac0926c04b02aadc48480b33c033d31bac00d6..59b0b7476009f53cb297e67bd342cc2b8df25bfd 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -83,7 +83,7 @@ class ASH_EXPORT DisplayManager
   //    the 2nd display. (Software Mirroring).
   // 3) UNIFIED mode creates single desktop across multiple displays.
   enum MultiDisplayMode {
-    EXTENDED,
+    EXTENDED = 0,
     MIRRORING,
     UNIFIED,
   };
@@ -297,6 +297,11 @@ class ASH_EXPORT DisplayManager
     return default_multi_display_mode_;
   }
 
+  // Reconfigure display configuration using the same
+  // physical display. TODO(oshima): Refactor and move this
+  // impl to |SetDefaultMultiDisplayMode|.
+  void ReconfigureDisplays();
+
   // Update the bounds of the display given by |display_id|.
   bool UpdateDisplayBounds(int64 display_id,
                            const gfx::Rect& new_bounds);