Introduce toSVGMPathElement(), and use it

As a step to change static_cast with toSVGXXX, static_cast<SVGMPathElement*> can
be changed with toSVGMPathElement().

Additionally this patch clean up static_cast<SVGUseElement*> using toSVGUseElement().

BUG=

Review URL: https://chromiumcodereview.appspot.com/21669006

git-svn-id: svn://svn.chromium.org/blink/trunk@155496 bbb929c8-8fbe-4397-9dbb-9b2b20218538