Oilpan: Replace RefPtr<Node> in V8DocumentCustom with an Oilpan transition type.

BUG=357163

Review URL: https://codereview.chromium.org/320233002

git-svn-id: svn://svn.chromium.org/blink/trunk@175762 bbb929c8-8fbe-4397-9dbb-9b2b20218538