[SVG2] Add support for the 'turn' unit in <angle>.

Spec:
https://svgwg.org/svg2-draft/painting.html#OrientAttribute
http://www.w3.org/TR/2012/WD-css3-values-20120308/#angles

BUG=377514

Review URL: https://codereview.chromium.org/303263008

git-svn-id: svn://svn.chromium.org/blink/trunk@176014 bbb929c8-8fbe-4397-9dbb-9b2b20218538