Support different reference files for virtual ref tests.

There are cases where a virtual test may produce different output than
the underlying base test. This is currently supported for pixel tests and
text-only tests, but not ref tests.

This patch makes ref tests work as well; the reference files will also
be run with the extra command line args from the virtual test suite,
but most references probably shouldn't need the arguments to produce the
correct output.

R=pdr@chromium.org
BUG=322782

Review URL: https://codereview.chromium.org/98253004

git-svn-id: svn://svn.chromium.org/blink/trunk@163727 bbb929c8-8fbe-4397-9dbb-9b2b20218538