Fix computed style of SVG fill/stroke properties on references

This is a merge of http://trac.webkit.org/changeset/154628 by
Rob Buis <rwlbuis@webkit.org>.

This patch changes the computed style of the fill and stroke properties
on references to include the url() function.

No new tests, updated existing tests to correctly match the feature.

BUG=232886

Review URL: https://chromiumcodereview.appspot.com/23614004

git-svn-id: svn://svn.chromium.org/blink/trunk@156870 bbb929c8-8fbe-4397-9dbb-9b2b20218538