Generate the isSVG*Element() / toSVG*Element() helper functions

Generate the isSVG*Element() / toSVG*Element() helper functions, similarly to
the HTML ones. This also makes the new Traversal<> API work for SVG Elements.

R=esprehn, pdr
BUG=346095

Review URL: https://codereview.chromium.org/189463005

git-svn-id: svn://svn.chromium.org/blink/trunk@168778 bbb929c8-8fbe-4397-9dbb-9b2b20218538