Use DEFINE_NODE_TYPE_CASTS instead of using manual toSVGFooElement()

Thanks to introduce DEFINE_NODE_TYPE_CASTS in r159446, we can replace manual
toSVGFooElement() with DEFINE_NODE_TYPE_CASTS(). It supports more various helper
functions as well as we can use it easily.

Thanks to use it, this cl removes unnecessary type casts.

BUG=N/A

Review URL: https://codereview.chromium.org/27096003

git-svn-id: svn://svn.chromium.org/blink/trunk@159516 bbb929c8-8fbe-4397-9dbb-9b2b20218538