[CSS Shapes] Serialize circle/ellipse positions

Circle/ellipse positions should always be present when serialized, and should
only have the 2 or 4-valued form. Keywords are converted to percentages
and simplified where possible. This patch adds some additional processing
that converts the parsed position into the serialized format, before
converting it to text. See http://dev.w3.org/csswg/css-shapes/#basic-shape-serialization.

Ported from:
https://bugs.webkit.org/show_bug.cgi?id=129404
https://bugs.webkit.org/show_bug.cgi?id=129726

BUG=322165

Review URL: https://codereview.chromium.org/185133006

git-svn-id: svn://svn.chromium.org/blink/trunk@168876 bbb929c8-8fbe-4397-9dbb-9b2b20218538