Save and restore unified mode

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 dc322087c2f1bb92d4198dae7d707ca427679bea..df966b2f93bc0d604d391433e2aa5fed89d48162 100644
--- a/ash/display/display_layout.h
+++ b/ash/display/display_layout.h
@@ -59,6 +59,9 @@ struct ASH_EXPORT DisplayLayout {
   // True if displays are mirrored.
   bool mirrored;
 
+  // True if multi displays should default to unified mode.
+  bool default_unified;
+
   // The id of the display used as a primary display.
   int64 primary_id;