Reviewed by Oliver Hunt.

        A little bit of MarkStack cleanup
        https://bugs.webkit.org/show_bug.cgi?id=56443
        
        Moved MarkStack functions into MarkStack.h/.cpp.
        
        SunSpider reports no change.

        * runtime/JSArray.h:
        * runtime/JSCell.h: Moved from here...
        * runtime/MarkStack.cpp:
        (JSC::MarkStack::markChildren):
        (JSC::MarkStack::drain): ...to here. Also, no need to inline drain. It's
        a huge function, and not called many times.

        * runtime/MarkStack.h:
        (JSC::MarkStack::~MarkStack): Moved near constructor, per style guide.
        (JSC::MarkStack::append):
        (JSC::MarkStack::deprecatedAppend):
        (JSC::MarkStack::internalAppend): Moved to here.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@81261 268f45cc-cd09-0410-ab3c-d52691b4dbfc