Free PerThreadAssertData when possible to avoid memory leak.

R=svenpanne@chromium.org
BUG=246567

Review URL: https://chromiumcodereview.appspot.com/16093024

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14947 ce2b1a6d-e550-0410-aec6-3dcde31c8c00