Add a few missing overrides found by a new clang warning.

Namely, -Winconsistent-missing-override. No behavior change.

BUG=v8:3658
LOG=N
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/688533002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24994 ce2b1a6d-e550-0410-aec6-3dcde31c8c00