Gardening: mark recently updated test as slow.

BUG=307721
TBR=antonm@chromium.org

Review URL: https://codereview.chromium.org/60733009

git-svn-id: svn://svn.chromium.org/blink/trunk@161511 bbb929c8-8fbe-4397-9dbb-9b2b20218538