Revert of Oilpan: Prepare to move RenderObject and RenderObjectChildList to Oilpan heap. (https://codereview.chromium.org/423093002/)

Reason for revert:
It seems it caused a lot of crashes in Oilpan bots.


Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179196

TBR=oilpan-reviews@chromium.org,zerny@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=398342

Review URL: https://codereview.chromium.org/425203002

git-svn-id: svn://svn.chromium.org/blink/trunk@179199 bbb929c8-8fbe-4397-9dbb-9b2b20218538