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

BUG=357163

Review URL: https://codereview.chromium.org/312073002

git-svn-id: svn://svn.chromium.org/blink/trunk@175453 bbb929c8-8fbe-4397-9dbb-9b2b20218538