Drastically reduce the number of created strings in d8.

This also reduces the number of TLS accesses, so it is a double-win. We have
230k less TLS accesses while running Octane now, so we are down to 23% now
compared to the start of these TLS-related CLs.

Introduced a tiny helper function for throwing on the way.

Review URL: https://codereview.chromium.org/11417029

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12991 ce2b1a6d-e550-0410-aec6-3dcde31c8c00