Fix JSON.stringifier's slow path wrt external strings.

R=verwaest@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12825016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14042 ce2b1a6d-e550-0410-aec6-3dcde31c8c00