[CSS Shapes] When the box value is set, derive radii from border-radius

Add support for BoxShape elliptical corners.

BUG=328433

Review URL: https://codereview.chromium.org/117783002

git-svn-id: svn://svn.chromium.org/blink/trunk@164201 bbb929c8-8fbe-4397-9dbb-9b2b20218538