Clean up ICU data tables on shutdown.

This is only used by d8 if compiled with external data tables, or an
embedder that uses external data tables and v8's version of ICU.

BUG=none
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/210973007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20283 ce2b1a6d-e550-0410-aec6-3dcde31c8c00