Reviewed by Darin Adler.

        Moved MarkedBlock data members to the head of the block
        https://bugs.webkit.org/show_bug.cgi?id=54482
        
        This allows for a variable-sized tail, to accommodate oversized blocks.

        SunSpider reports no change.
        
        * runtime/JSCell.h:
        (JSC::JSCell::MarkedBlock::allocate):
        * runtime/MarkedBlock.cpp:
        (JSC::MarkedBlock::destroy):
        (JSC::MarkedBlock::MarkedBlock):
        (JSC::MarkedBlock::sweep):
        * runtime/MarkedBlock.h: Added missing element to the CELLS_PER_BLOCK
        calculation. This kind of error is why we want to migrate to the system
        described below.

        (JSC::roundUpToMultipleOf):
        (JSC::MarkedBlock::firstCell):
        (JSC::MarkedBlock::cells):
        (JSC::MarkedBlock::cellNumber): Use subtraction instead of masking to
        calculate cell number. The mask is no longer correct because the first
        cell is not at the head of the block.

        (JSC::MarkedBlock::forEach): Replaced m_cells data member with a cells()
        accessor. We want to use sizeof(MarkedBlock) to calculate the size of the
        block header, so we can't have an explicit data member to represent the block tail.
        
        Also replaced iteration from zero with iteration from startCell(), since
        the first N cells are now occupied by the header.

        * runtime/MarkedSpace.cpp:
        (JSC::MarkedSpace::MarkedSpace):
        (JSC::MarkedSpace::reset): Replaced iteration from zero as above.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78605 268f45cc-cd09-0410-ab3c-d52691b4dbfc