Oilpan: Replace RefPtr<Node> in core/svg/ with Oilpan transition types.

BUG=357163

Review URL: https://codereview.chromium.org/311193007

git-svn-id: svn://svn.chromium.org/blink/trunk@175529 bbb929c8-8fbe-4397-9dbb-9b2b20218538