[CSS Shapes] serialization of the computed value should omit the default radii

We should omit closest-side value, wherever is possible. Specification:
​http://www.w3.org/TR/2014/CR-css-shapes-1-20140320/#basic-shape-computed-values
This change aligns the behavior to the specification.
Added new test cases for the shape-outside parsing tests.

This is a merge of http://trac.webkit.org/changeset/169406 by Zoltan Horvath <zoltan@webkit.org>.

BUG=371934

Review URL: https://codereview.chromium.org/300843011

git-svn-id: svn://svn.chromium.org/blink/trunk@175085 bbb929c8-8fbe-4397-9dbb-9b2b20218538