Use DEFINE_NODE_TYPE_CASTS instead of using manual toSVGFooElement()

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159516

[gyuyoung-inactive, 2013-10-12 10:10:21]

Add tkent.

[tkent, 2013-10-12 12:48:24]

lgtm

[commit-bot: I haz the power, 2013-10-12 12:48:33]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/gyuyoung.kim@samsung.com/27096003/1

[commit-bot: I haz the power, 2013-10-12 14:16:26]

Change committed as 159516