Reviewed by Oliver Hunt.

        Moved the "nextAtom" allocation pointer into MarkedBlock for better encapsulation
        https://bugs.webkit.org/show_bug.cgi?id=55079
        
        SunSpider reports no change.

        * runtime/Heap.cpp:
        (JSC::Heap::reset): Moved Zombie sweeping here, up from MarkedSpace,
        since we want Heap to logically control MarkedSpace. MarkedSpace should
        never choose to sweep itself.

        * runtime/JSCell.h:
        (JSC::JSCell::MarkedBlock::allocate): Updated for nextAtom becoming a
        member of MarkedBlock. No need to reset nextAtom to firstAtom() when
        we reach the end of a block, since there's now an explicit reset pass
        during GC.

        * runtime/MarkedBlock.cpp:
        (JSC::MarkedBlock::MarkedBlock):
        * runtime/MarkedBlock.h:
        (JSC::MarkedBlock::reset): Added the nextAtom data member, and reordered
        some data members to improve cache locality.

        * runtime/MarkedSpace.cpp:
        (JSC::MarkedSpace::MarkedSpace):
        (JSC::MarkedSpace::allocate):
        (JSC::MarkedSpace::reset):
        * runtime/MarkedSpace.h:
        (JSC::CollectorHeap::CollectorHeap): Removed nextAtom, and added an
        explicit reset pass.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@79492 268f45cc-cd09-0410-ab3c-d52691b4dbfc