Reviewed by Oliver Hunt.

        Introduced the concept of opaque roots, in preparation for marking the DOM with them
        https://bugs.webkit.org/show_bug.cgi?id=57903

        * JavaScriptCore.exp: Who likes export files? I do!

        * collector/handles/HandleHeap.cpp:
        (JSC::isValidWeakHandle): Factored out a helper function for ASSERTs.

        (JSC::WeakHandleOwner::~WeakHandleOwner): Moved from header to avoid
        weak linkage problems.

        (JSC::WeakHandleOwner::isReachableFromOpaqueRoots): New callback.
        Currently unused.

        (JSC::WeakHandleOwner::finalize): Switched from pure virtual to a
        default empty implementation, since not all clients necessarily want
        or need non-trivial finalizers.

        (JSC::HandleHeap::markWeakHandles): Split updateWeakHandles into two
        passes. The first pass marks all reachable weak handles. The second pass
        finalizes all unreachable weak handles. This must be two passes because
        we don't know the set of finalizable weak handles until we're done
        marking all weak handles.

        (JSC::HandleHeap::finalizeWeakHandles): Use new helper function.

        * collector/handles/HandleHeap.h: Ditto.

        * runtime/Heap.cpp: 
        (JSC::Heap::destroy):
        (JSC::Heap::markRoots):
        (JSC::Heap::reset): Split out handle marking from handle finalization.

        * runtime/MarkStack.cpp:
        (JSC::MarkStack::reset):
        * runtime/MarkStack.h:
        (JSC::MarkStack::addOpaqueRoot):
        (JSC::MarkStack::containsOpaqueRoot):
        (JSC::MarkStack::opaqueRootCount):
        (JSC::HeapRootMarker::markStack): New helper functions for managing the
        set of opaque roots.

        * runtime/WeakGCMap.h:
        (JSC::WeakGCMap::finalize): Renamed to match parent class declaration.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83011 268f45cc-cd09-0410-ab3c-d52691b4dbfc