Reviewed by Oliver Hunt.

        REGRESSION [r78794-r79249] Allocation of memory is slow when number of active objects is large
        https://bugs.webkit.org/show_bug.cgi?id=56823
        
        Partial fix for most of the problem. (TOT still shows a regression, though.)

        * runtime/Heap.cpp:
        (JSC::Heap::reportExtraMemoryCostSlowCase): Use highWaterMark(), instead of
        capacity(), since capacity() is O(n) relative to the size of the heap.

        In limited circumstances, capacity() is also worse than highWaterMark()
        for measuring extra cost relative to heap size, since capacity() only
        measures the *current* capacity of the heap, but the heap will grow if
        necessary to attain highWaterMark().


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@82162 268f45cc-cd09-0410-ab3c-d52691b4dbfc