Remove SVGAnimatedType

With the transition to the new SVGProperty hierarchy complete,
SVGAnimatedType is nothing but a wrapper holding a reference to
a NewSVGPropertyBase.
By unwrapping this reference in SVGAnimateElement, and modifying
the SVGAnimatedTypeAnimator interface to pass NewSVGPropertyBase
references instead it becomes possible to remove SVGAnimatedType
without much hassle.

The SVGAnimatedTypeAnimator interfaces are to a large degree left
unchanged, with the exceptions being:

 * addAnimatedTypes is folded in favor just using the add() method
   on NewSVGPropertyBase.

 * resetAnimValToBaseVal returns the value instead of setting it
   through an out-parameter.

Review URL: https://codereview.chromium.org/183143002

git-svn-id: svn://svn.chromium.org/blink/trunk@168150 bbb929c8-8fbe-4397-9dbb-9b2b20218538