Reviewed by Oliver Hunt.

        Drain the mark stack while marking weak handles, not after.
        https://bugs.webkit.org/show_bug.cgi?id=58574

        Otherwise, items that would have caused more weak handle marking are
        processed after all weak handle marking has finished, and referenced
        weak handles get recycled.

        * heap/HandleHeap.cpp:
        (JSC::HandleHeap::markWeakHandles): Removed looping from here, since we
        want Heap::markRoots to be responsible for draining the mark stack.

        * heap/Heap.cpp:
        (JSC::Heap::markRoots): Moved looping to here, as explained above.
        
        For efficiency's sake, drain the mark stack before starting to mark weak
        handles. Otherwise, items drained while marking weak handles may force
        an extra trip through the weak handle list.

        For correctness's sake, drain the mark stack each time through the weak
        handle list. Otherwise, opaque roots that would make weak handles reachable
        are not discovered until after weak handle marking is over.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83910 268f45cc-cd09-0410-ab3c-d52691b4dbfc