Oilpan: Replace RefPtrs to Node and its subclasses in core/loader with Oilpan transition types.

This CL makes DocumentWriter GarbageCollectedFinalized.

BUG=357163

Review URL: https://codereview.chromium.org/314953002

git-svn-id: svn://svn.chromium.org/blink/trunk@175461 bbb929c8-8fbe-4397-9dbb-9b2b20218538