Reviewed by Oliver Hunt.

        A little more encapsulation for MarkedBlock: Moved allocate() and sweep() into MarkedBlock
        https://bugs.webkit.org/show_bug.cgi?id=54253
        
        SunSpider reports no change.

        * runtime/CollectorHeapIterator.h: Removed DeadObjectIterator, since it
        is now unused.

        * runtime/Heap.cpp:
        (JSC::Heap::reset): Moved the call to shrink() here, since it seems a
        little more clear for MarkedSpace's client to tell it explicitly when to
        shrink.

        * runtime/JSCell.h:
        (JSC::JSCell::MarkedBlock::allocate): Split out from MarkedSpace::allocate.

        * runtime/MarkedBlock.cpp:
        (JSC::MarkedBlock::sweep): Split out from MarkedSpace::sweep, and
        converted to more directly iterate a MarkedBlock based on knowing its
        internal structure.

        * runtime/MarkedBlock.h:
        * runtime/MarkedSpace.cpp:
        (JSC::MarkedSpace::allocate):
        (JSC::MarkedSpace::sweep):
        * runtime/MarkedSpace.h: Split out the code mentioned above.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78284 268f45cc-cd09-0410-ab3c-d52691b4dbfc