Drop SVGElement::isGradientStop() virtual function

Drop SVGElement::isGradientStop() virtual function and use the isSVGStopElement()
helper function instead, for consistency. Unless I am missing something, we don't
need a virtual function here and checking for the SVGNames::stopTag should be
sufficient.

R=pdr
BUG=346095

Review URL: https://codereview.chromium.org/192103002

git-svn-id: svn://svn.chromium.org/blink/trunk@168795 bbb929c8-8fbe-4397-9dbb-9b2b20218538