Add --force-compositing-mode to virtual/gpu/canvas/philip

I get no failures on linux with this flag.

R=enne, ojan
BUG=361729

Review URL: https://codereview.chromium.org/279983002

git-svn-id: svn://svn.chromium.org/blink/trunk@173776 bbb929c8-8fbe-4397-9dbb-9b2b20218538