Add a UseCounter for SVGUseElement.instanceRoot

This patch adds a UseCounter for instanceRoot, the accessor for the
SVG instance tree. This patch does not add a usecounter for
SVGUseElement.animatedInstanceRoot because it is not suported
(see: SVGUseElement::animatedInstanceRoot)

BUG=313438

Review URL: https://codereview.chromium.org/54053002

git-svn-id: svn://svn.chromium.org/blink/trunk@161021 bbb929c8-8fbe-4397-9dbb-9b2b20218538