Mark selection-gaps-at-fractional-offsets.html as flaky on win/mac

TBR=leviw@chromium.org

Review URL: https://codereview.chromium.org/64633008

git-svn-id: svn://svn.chromium.org/blink/trunk@161737 bbb929c8-8fbe-4397-9dbb-9b2b20218538