Oilpan: Prepare to move RenderObject and RenderObjectChildList to Oilpan heap.

We have a lot of remaining raw pointer members to RenderObject. Follow-up
patches will resolve them.

BUG=398342

Review URL: https://codereview.chromium.org/423093002

git-svn-id: svn://svn.chromium.org/blink/trunk@179196 bbb929c8-8fbe-4397-9dbb-9b2b20218538