Also allow the empty object map to keep transitions.

With the old implementation, due to the map-check being inadequate, such
transitions were already added for cross-context field stores. It is not
necessary anymore to not store transitions, since we properly clear
non-live transitions. Globally enabling this feature will help find more
bugs.

BUG=v8:2518
R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12092063

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13558 ce2b1a6d-e550-0410-aec6-3dcde31c8c00