Gardening: Widen flaky expectation for fast/text/aat-morx.html

BUG=418091
TBR=eustas@chromium.org,eae@chromium.org

Review URL: https://codereview.chromium.org/622133002

git-svn-id: svn://svn.chromium.org/blink/trunk@183156 bbb929c8-8fbe-4397-9dbb-9b2b20218538