[CSS Shapes] inset does not properly clamp large corner radii

Make sure that radii that are larger than 50% of the associated shape
dimension (height or width) are properly reduced the same way as
border-radius radii are.

Ported over from http://trac.webkit.org/changeset/165261.

BUG=351775

Review URL: https://codereview.chromium.org/195793009

git-svn-id: svn://svn.chromium.org/blink/trunk@169086 bbb929c8-8fbe-4397-9dbb-9b2b20218538