Fix breakage of mac-retina port in run-webkit-tests after r163814.

TBR=adamk@chromium.org
BUG=322749

Review URL: https://codereview.chromium.org/114533003

git-svn-id: svn://svn.chromium.org/blink/trunk@163817 bbb929c8-8fbe-4397-9dbb-9b2b20218538