Gardening: Mark one test as flaky.

BUG=284531
TBR=alexis.menard@intel.com

Review URL: https://codereview.chromium.org/23724011

git-svn-id: svn://svn.chromium.org/blink/trunk@157219 bbb929c8-8fbe-4397-9dbb-9b2b20218538