Suppress a flaky test

BUG=317556
TBR=eae@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/66893004

git-svn-id: svn://svn.chromium.org/blink/trunk@161719 bbb929c8-8fbe-4397-9dbb-9b2b20218538