Disable fast/event/hit-test-counts.html on Mac.

This test is very sensitive to layout changes.

BUG=430980
TBR=rbyers@chromium.org

Review URL: https://codereview.chromium.org/707013002

git-svn-id: svn://svn.chromium.org/blink/trunk@184932 bbb929c8-8fbe-4397-9dbb-9b2b20218538