Mark png-suite/test.html tests as flaky on Mac

BUG=311429
TBR=falken@chromiun.org

Review URL: https://codereview.chromium.org/41703004

git-svn-id: svn://svn.chromium.org/blink/trunk@160507 bbb929c8-8fbe-4397-9dbb-9b2b20218538