Mark compositing/images/direct-pdf-image.html as flaky on Mac

TBR=tony@chromium.org
BUG=365400

Review URL: https://codereview.chromium.org/243583009

git-svn-id: svn://svn.chromium.org/blink/trunk@172071 bbb929c8-8fbe-4397-9dbb-9b2b20218538