SVGRenderStyle accessors should return references.
<https://webkit.org/b/123656>

RenderStyle::svgStyle() and accessSVGStyle() never return null,
so make them return references instead.

This flushed out a myriad of pointless null checks and assertions.

Reviewed by Anders Carlsson.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158484 268f45cc-cd09-0410-ab3c-d52691b4dbfc