Fix computation of a percentage radius for circle shape-outside

This brings the implementaion in line with the current CSS Shapes spec.

Also update the nested container with unresolved height test because with a
circle it isn't really testing just for unresolved height behavior.

BUG=299188

Review URL: https://chromiumcodereview.appspot.com/24868002

git-svn-id: svn://svn.chromium.org/blink/trunk@158462 bbb929c8-8fbe-4397-9dbb-9b2b20218538