[CSS Shapes] Add BoxShape unit tests

This unit test covers as much of BoxShape as has been integrated so far.

BUG=328221

Review URL: https://codereview.chromium.org/100723004

git-svn-id: svn://svn.chromium.org/blink/trunk@163896 bbb929c8-8fbe-4397-9dbb-9b2b20218538