Move RenderSVGResource::cast() to RenderSVGResourceContainer

Only use with/called on the latter.

BUG=420022

Review URL: https://codereview.chromium.org/630103002

git-svn-id: svn://svn.chromium.org/blink/trunk@183323 bbb929c8-8fbe-4397-9dbb-9b2b20218538