Add a suppression for new test  compositing/update-paint-phases.html.

This test seems to give different text results on some Mac platforms:
(drawsContent 1) missing on some layers. (This may be correct for
rebaselining, but I wanted Ian to take a look first.)

TBR=vollick@chromium.org
BUG=361940

Review URL: https://codereview.chromium.org/251083002

git-svn-id: svn://svn.chromium.org/blink/trunk@172712 bbb929c8-8fbe-4397-9dbb-9b2b20218538