Update percentage rectangle and inset rectangle tests

Update percentage based tests to work with the new positioning model.
Also move the tests to be W3C spec tests, so they have been impored
back here from the CSSWG repo.

This was committed to WebKit as http://trac.webkit.org/changeset/153437

BUG=260357
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/21378004

git-svn-id: svn://svn.chromium.org/blink/trunk@155272 bbb929c8-8fbe-4397-9dbb-9b2b20218538