New positioning model: support for stacked floats with shape-outside

Import new test for stacked floats from the CSSWG's repository.

BUG=260357
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/21645002

git-svn-id: svn://svn.chromium.org/blink/trunk@155444 bbb929c8-8fbe-4397-9dbb-9b2b20218538