Remove RenderSVGResourceContainer::cast()

Fold into getRenderSVGResourceById<Renderer>.

BUG=420022

Review URL: https://codereview.chromium.org/631233002

git-svn-id: svn://svn.chromium.org/blink/trunk@183484 bbb929c8-8fbe-4397-9dbb-9b2b20218538