Correctly expand literal buffer for surrogate pairs.

R=jkummerow@chromium.org
BUG=chromium:423212
LOG=Y

Review URL: https://codereview.chromium.org/652743005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24603 ce2b1a6d-e550-0410-aec6-3dcde31c8c00