Support sharing maps after an changing an object's ElementsKind for element kinds other then external elements.

R=svenpanne@chromium.org
BUG=none
TEST=external-arrays.js

Review URL: http://codereview.chromium.org/7862036

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9304 ce2b1a6d-e550-0410-aec6-3dcde31c8c00