Add --disable-gpu-compositing flag to virtual software compositing tests

https://codereview.chromium.org/19267016/ changes the meaning of --enable-software-compositing to enable software compositing, but not force it, so the virtual test needs to pass the --disable-gpu-compositing flag to ensure that it's not using the GPU instead.

BUG=

Review URL: https://chromiumcodereview.appspot.com/20890002

git-svn-id: svn://svn.chromium.org/blink/trunk@155133 bbb929c8-8fbe-4397-9dbb-9b2b20218538
New positioning model: support for circle and ellipse shape-outside

Update circle and ellipse tests to work with new positioning. These
have been re-homed as W3C spec tests, and thus are imported from the
CSSWG's repository.

This is a port from WebKit http://trac.webkit.org/changeset/153337

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/21104005

git-svn-id: svn://svn.chromium.org/blink/trunk@155134 bbb929c8-8fbe-4397-9dbb-9b2b20218538