Fix Retina expectation for fast/events/touch/gesture/gesture-tap-frame-scrollbar.html

TBR=caseq@chromium.org
BUG=414034

Review URL: https://codereview.chromium.org/576213002

git-svn-id: svn://svn.chromium.org/blink/trunk@182171 bbb929c8-8fbe-4397-9dbb-9b2b20218538