Oilpan: Add a test to confirm that pre-finalization hook can be implmemented with HeapHashSet<WeakMmber<T>, U>.

BUG=

Review URL: https://codereview.chromium.org/204313002

git-svn-id: svn://svn.chromium.org/blink/trunk@169821 bbb929c8-8fbe-4397-9dbb-9b2b20218538