Adds RefPtr::leakRef method to allow raw pointers that prevent destruct.
Part of the removal of PassRefPtr.
Adds RefPtr::leakRef method which returns a raw pointer while
maintaining a ref to the RefCounted object. This creates a hanging ref
on the object.
RefPtr::leakRef is intended to replace PassRefPtr::leakRef.
Also replaces all conversions of RefPtr objects to PassRefPtr objects
in order to use PassRefPtr::leakRef (i.e. through .release().leakRef())
with direct usage of the new RefPtr::leakRef method.