[CSS Shapes] Add parsing support for gradients

Added support for shape-outside gradient values.

This is a merge of http://trac.webkit.org/changeset/167518 by Hans Muller <hmuller@adobe.com>.

BUG=322165

Review URL: https://codereview.chromium.org/244693003

git-svn-id: svn://svn.chromium.org/blink/trunk@172790 bbb929c8-8fbe-4397-9dbb-9b2b20218538