Reviewed by Sam Weinig.

        Removed the invariant that the last cell in a block is always marked
        https://bugs.webkit.org/show_bug.cgi?id=54713
        
        SunSpider reports no change.
        
        This adds one branch to allocation, but simplifies the mark invariant,
        especially in a world of variable-sized cells. Now, it really is true
        that any cell whose mark bit is set is a valid, live cell whose
        constructor has run and whose destructor has not run.

        * runtime/JSCell.h: 
        (JSC::JSCell::MarkedBlock::allocate): Changed this do-while into a while
        since we can no longer rely on a set mark bit to break out of this loop
        before it reaches the end of the block.

        * runtime/MarkedBlock.cpp:
        (JSC::MarkedBlock::MarkedBlock):
        (JSC::MarkedBlock::sweep): 
        * runtime/MarkedBlock.h:
        (JSC::MarkedBlock::isEmpty):
        (JSC::MarkedBlock::clearMarks):
        (JSC::MarkedBlock::markCount):
        (JSC::MarkedBlock::forEach): No need to set a special last mark bit.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78954 268f45cc-cd09-0410-ab3c-d52691b4dbfc