Use SVGElement as parameter for isDirectReference
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@179515 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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