Return MaybeHandle from NewExternalStringFrom*.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/223653003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20481 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge