Bug 901746 - Part 1: Fix whitespace in xpcom's weakref code.
Bug 901746 - Part 2: Add finalizers to weak references.
Bug 901746 - Part 3: Add mozilla::WeakSet, a self-compacting set of weakrefs.
Bug 901753 - Switch nsObserverList to use WeakSet.
Bug 901759 - Add {add,remove}WeakMessageListener to nsFrameMessageManager.
Switch DOMRequestHelper over to using weak message listeners.