New positioning model: support for polygon shape-outside

Update polygon tests to work for new positioning model and migrate
them to be W3C spec tests. Thus, they have been imported from the
CSSWG repository now.

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/21573004

git-svn-id: svn://svn.chromium.org/blink/trunk@155398 bbb929c8-8fbe-4397-9dbb-9b2b20218538