Add --force-compositing-mode to virtual/threaded and virtual/stable.

The threaded suite is already FCM so this is a no-op but just to be
consistent. The stable suite shows no failures on linux with this
flag.

R=enne, ojan
BUG=361729

Review URL: https://codereview.chromium.org/279963002

git-svn-id: svn://svn.chromium.org/blink/trunk@173775 bbb929c8-8fbe-4397-9dbb-9b2b20218538