Apply DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION to HTMLUnknownElement

DEFINE_NODE_TYPE_CASTS_WITH_FUNCTION supports more plenty helper functions than
manual toFoo().

BUG=309516

Review URL: https://codereview.chromium.org/98443003

git-svn-id: svn://svn.chromium.org/blink/trunk@162965 bbb929c8-8fbe-4397-9dbb-9b2b20218538