Drop SVGElement::isSVGSVGElement() virtual function

Drop SVGElement::isSVGSVGElement() virtual function and use the
isSVGSVGElement(element) helper function instead for consistency.
Unless I am missing something, we don't need a virtual function for
this and checking for the SVGNames::svgTag tag should be sufficient.

R=pdr
BUG=346095

Review URL: https://codereview.chromium.org/192093002

git-svn-id: svn://svn.chromium.org/blink/trunk@168794 bbb929c8-8fbe-4397-9dbb-9b2b20218538