Introduce toSVGRectElement(), use it

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

BUG=

Review URL: https://chromiumcodereview.appspot.com/19781008

git-svn-id: svn://svn.chromium.org/blink/trunk@154897 bbb929c8-8fbe-4397-9dbb-9b2b20218538