Define DEBUG for v8_optimized_debug=2

Thereby ensuring there is only a minimal performance regression vs. NDEBUG (now it's only about 10% slower rather than ~2x).

R=jkummerow@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/39183004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17392 ce2b1a6d-e550-0410-aec6-3dcde31c8c00