Gardening: styles-add-blank-property.html is flaky

BUG=419531
TBR=eustas@chromium.org,pfeldman@chromium.org

Review URL: https://codereview.chromium.org/618833006

git-svn-id: svn://svn.chromium.org/blink/trunk@183091 bbb929c8-8fbe-4397-9dbb-9b2b20218538