Add V8_FINAL and V8_OVERRIDE macros for C++11 final/override.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/22914019

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16231 ce2b1a6d-e550-0410-aec6-3dcde31c8c00