[mac] Update fast/images/png-suite/test.html TestExpectations

Passing without flakes since r273650 by jbauman@chromium.org,
so update [mac] TestExpectations for the png-suite test.

BUG=311429

Review URL: https://codereview.chromium.org/596453002

git-svn-id: svn://svn.chromium.org/blink/trunk@182461 bbb929c8-8fbe-4397-9dbb-9b2b20218538