remove V8_USE_UNSAFE_HANDLES define

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/16896004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15142 ce2b1a6d-e550-0410-aec6-3dcde31c8c00