Let RenderStyle::svgStyle() return reference

This seems the last accessor of RenderStyle DataRef fields returning
pointer. Let it return reference because DataRef is never null, also
to keep consistent with other accessors.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/381473003

git-svn-id: svn://svn.chromium.org/blink/trunk@177783 bbb929c8-8fbe-4397-9dbb-9b2b20218538