Do not use --allow-webui-compositing for virtual test suite

This flag was removed in https://codereview.chromium.org/167713003
and so it should not be passed in to any virtual test suite.

BUG=none

Review URL: https://codereview.chromium.org/176823006

git-svn-id: svn://svn.chromium.org/blink/trunk@167961 bbb929c8-8fbe-4397-9dbb-9b2b20218538