Fix webexposed/css-properties-as-js-properties.html after r156571

Trivial test fix for a missing property after r156571 landed.

TBR=ed@opera.com
BUG=223766

Review URL: https://codereview.chromium.org/22999040

git-svn-id: svn://svn.chromium.org/blink/trunk@156573 bbb929c8-8fbe-4397-9dbb-9b2b20218538