Use DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION to generate toSVGSMILElement()

DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION supports more plenty toFoo helper functions than
manual toFoo(). This cl changes isSMILElement from static member to inline function to generate
toSVGSMILElement().

BUG=309516

Review URL: https://codereview.chromium.org/103393002

git-svn-id: svn://svn.chromium.org/blink/trunk@163161 bbb929c8-8fbe-4397-9dbb-9b2b20218538