Mark test as flaky.

BUG=418483
NOTRY=true
TBR=lushnikov@chromium.org

Review URL: https://codereview.chromium.org/613653002

git-svn-id: svn://svn.chromium.org/blink/trunk@182833 bbb929c8-8fbe-4397-9dbb-9b2b20218538