Introduce DEFINE_RENDER_SVG_RESOURCE_TYPE_CASTS, and use it

As a step to let toFoo use DEFINE_TYPE_CASTS, this cl introduce DEFINE_RENDER_SVG_RESOURCE_TYPE_CASTS,
which can support more helper functions rather than manual toFoo(). This cl will help to detect
bad type casts further.

BUG=309516

Review URL: https://codereview.chromium.org/59843012

git-svn-id: svn://svn.chromium.org/blink/trunk@161431 bbb929c8-8fbe-4397-9dbb-9b2b20218538