[Sheriff] Mark profviz flaky on GC stress.

BUG=
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/149763002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18929 ce2b1a6d-e550-0410-aec6-3dcde31c8c00