Move isValid implementations into SVGGraphicsElement

All subclasses share the same implementation, so provide
it in SVGGraphicsElement.

Review URL: https://codereview.chromium.org/139813003

git-svn-id: svn://svn.chromium.org/blink/trunk@165175 bbb929c8-8fbe-4397-9dbb-9b2b20218538