[CSS Shapes] inset args and radial args should serialize to the simplest form

Add code for suppressing serializing of duplicate values. The implementation for the border-radius
part is inspired by valueForBorderRadiusShorthand.

BUG=350092

Review URL: https://codereview.chromium.org/227793002

git-svn-id: svn://svn.chromium.org/blink/trunk@171089 bbb929c8-8fbe-4397-9dbb-9b2b20218538