Reviewed by Sam Weinig.

        Fixed some math errors when when using variable-sized cells
        https://bugs.webkit.org/show_bug.cgi?id=54717
        
        SunSpider reports no change.
        
        Computer Science Barbie says, "Math is not so hard afterall!"

        * runtime/JSCell.h:
        (JSC::JSCell::MarkedBlock::allocate): Round up when calculating the
        minimum number of atoms required for a cell, since rounding down
        will get you splinched.

        * runtime/MarkedBlock.cpp:
        (JSC::MarkedBlock::MarkedBlock):
        (JSC::MarkedBlock::sweep):
        * runtime/MarkedBlock.h:
        (JSC::MarkedBlock::forEach): Changed a bunch of != tests to < tests
        because m_endAtom is actually a fuzzy end -- iterating from firstAtom()
        may not hit m_endAtom exactly.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78957 268f45cc-cd09-0410-ab3c-d52691b4dbfc