Gardening: svg/custom tests are flaky on Mac

svg/custom/getSubStringLength.html
svg/custom/font-face-cascade-order.svg

BUG=311469
TBR=jsbell

Review URL: https://codereview.chromium.org/42063004

git-svn-id: svn://svn.chromium.org/blink/trunk@160531 bbb929c8-8fbe-4397-9dbb-9b2b20218538