Mark tests that fail on Retina devices as flaky until they are properly supported.

TBR=ojan@chromium.org, abarth@chromium.org
BUG=292886

Review URL: https://codereview.chromium.org/23625014

git-svn-id: svn://svn.chromium.org/blink/trunk@157858 bbb929c8-8fbe-4397-9dbb-9b2b20218538