[Sheriff] Temporarily mark two mjsunit tests as flaky.

The tests are blocking the v8 lkgr. They will be unmarked again after there is infrastructure to disable them on GC stress only.

TBR=mvstanton@chromium.org
BUG=

Review URL: https://codereview.chromium.org/139343008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18698 ce2b1a6d-e550-0410-aec6-3dcde31c8c00