Use SVGElement as parameter for isDirectReference

It is a bit more efficient to use SVGElement for the isSVG checks, so
make isDirectReference take a SVGElement parameter. Also add an early
return in SVGUseElement::toClipPath.

Review URL: https://codereview.chromium.org/440713002

git-svn-id: svn://svn.chromium.org/blink/trunk@179515 bbb929c8-8fbe-4397-9dbb-9b2b20218538