[Sheriff] Mark flaky webkit test.

TBR=jkummerow@chromium.org
BUG=v8:2989

Review URL: https://codereview.chromium.org/62283008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17686 ce2b1a6d-e550-0410-aec6-3dcde31c8c00