Mark some tests as flaky.

These are killing the blink CQ.

BUG=374569,374572
TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/289303002

git-svn-id: svn://svn.chromium.org/blink/trunk@174238 bbb929c8-8fbe-4397-9dbb-9b2b20218538