[Sheriff] Mark new regression test as flaky.

BUG=336820
LOG=n
R=bmeurer@chromium.org
TBR=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/139923007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18990 ce2b1a6d-e550-0410-aec6-3dcde31c8c00