Reviewed by Oliver Hunt.

        Removed some complexity from HandleHeap
        https://bugs.webkit.org/show_bug.cgi?id=57650
        
        Eliminated pointer-tagging flags.
        
        Tied being weak to having a finalizer (or at least a finalizer sentinel).

        * collector/handles/HandleHeap.cpp:
        (JSC::HandleHeap::clearWeakPointers): Removed the special self-destroying
        flag. It was unused. If we bring it back, we'll probably use a shared
        autodeallocating finalizer instead.

        * collector/handles/HandleHeap.h:
        (JSC::HandleHeap::makeWeak): makeWeak and adding a finalizer are now
        a single, atomic operation -- this makes the relationship between
        finalizers and weak pointers clearer, and impossible to get wrong.

        (JSC::HandleHeap::Node::Node):
        (JSC::HandleHeap::Node::handleHeap): No more flags.

        (JSC::HandleHeap::Node::makeWeak):
        (JSC::HandleHeap::Node::isWeak): Ditto above. We use a special sentienl
        value in the finalizer slot to indicate that a handle is weak but doesn't
        require an external function call for finalization.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@82718 268f45cc-cd09-0410-ab3c-d52691b4dbfc