Generate toSVGAnimateElement to support more plenty helper functions

We can use more various toFoo() functions when generating toFoo(). This cl add
isSVGAnimateElement() to use DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION.

BUG=309516

Review URL: https://codereview.chromium.org/106023002

git-svn-id: svn://svn.chromium.org/blink/trunk@163208 bbb929c8-8fbe-4397-9dbb-9b2b20218538