Improve SeqString::Truncate for latest allocated strings.

R=hpayer@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12440061

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14088 ce2b1a6d-e550-0410-aec6-3dcde31c8c00