Stop using SVGElementInstance in SVGElement

Stop using SVGElementInstance in SVGElement since we want to remove it.

BUG=313438

Review URL: https://codereview.chromium.org/272623003

git-svn-id: svn://svn.chromium.org/blink/trunk@173637 bbb929c8-8fbe-4397-9dbb-9b2b20218538