Oilpan: Make sure we can make pre-finalization hook with Visitor::registerWeakMembers.

BUG=none
TEST=automated

Review URL: https://codereview.chromium.org/199883002

git-svn-id: svn://svn.chromium.org/blink/trunk@169307 bbb929c8-8fbe-4397-9dbb-9b2b20218538