Reviewed by Oliver Hunt.

        Optimized ConservativeSet to avoid double-visiting objects
        https://bugs.webkit.org/show_bug.cgi?id=61592
        
        SunSpider thinks this might be a 1% speedup

        * heap/ConservativeRoots.h:
        (JSC::ConservativeRoots::add): Use testAndClearMarked to avoid double-visiting
        an object.

        * heap/Heap.h:
        (JSC::Heap::isMarked):
        (JSC::Heap::testAndSetMarked):
        (JSC::Heap::testAndClearMarked):
        (JSC::Heap::setMarked): Added testAndClearMarked. Changed argument type
        to void*, since clients want to ask questions about arbitrary pointers
        into the heap, even when they aren't known to be JSCells.

        * heap/MarkedBlock.h:
        (JSC::MarkedBlock::testAndClearMarked):
        * heap/MarkedSpace.h:
        (JSC::MarkedSpace::isMarked):
        (JSC::MarkedSpace::testAndSetMarked):
        (JSC::MarkedSpace::testAndClearMarked):
        (JSC::MarkedSpace::setMarked):
        (JSC::MarkedSpace::contains): Ditto.

        * wtf/Bitmap.h:
        (WTF::::testAndClear): New function for ConservativeRoots's inverted
        marking pass.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87522 268f45cc-cd09-0410-ab3c-d52691b4dbfc