Make block writes in the serializer more efficient.

And also fix a OOB read in SerializeExternalString.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/671633004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24858 ce2b1a6d-e550-0410-aec6-3dcde31c8c00