Simplify SVGAnimat\w+Element::parseAttribute

- |SVGAnimate{Motion,Transform}Element::isSupportedAttribute| is only
called from |SVGAnimate{Motion,Transform}Element::parseAttribute|. Inlined.

BUG=397902

Review URL: https://codereview.chromium.org/424733002

git-svn-id: svn://svn.chromium.org/blink/trunk@179118 bbb929c8-8fbe-4397-9dbb-9b2b20218538