Allow the morphing of strings to external strings to avoid having to

Allow the morphing of strings to external strings to avoid having to
create copies in the embedding code (aka WebKit V8 bindings) on every
external use.

Review URL: http://codereview.chromium.org/21117

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1252 ce2b1a6d-e550-0410-aec6-3dcde31c8c00