Fix the fast/sub-pixel/ellipsis-zoom.html reftest

I broke this in r155341 aka https://codereview.chromium.org/21414002/

R=ojan@chromium.org
BUG=259083

Review URL: https://codereview.chromium.org/20751007

git-svn-id: svn://svn.chromium.org/blink/trunk@155445 bbb929c8-8fbe-4397-9dbb-9b2b20218538