Animate SVG's offset and stroke properties

We resolve the naming collision between Web Aniations's keyframe
'offset' and SVG's 'offset' attribute
http://www.w3.org/TR/SVG/pservers.html#StopElementOffsetAttribute
by using the name 'svgOffset' in the keyframe properties dictionary
passed to Web Animations.

Note that we do not yet animate stroke-dasharray.