Reviewed by Oliver Hunt.

        Removed some interdependency between Heap and SmallStrings by simplifying
        the SmallStrings lifetime model
        https://bugs.webkit.org/show_bug.cgi?id=61579
        
        SunSpider reports no change.
        
        Using Weak<T> could accomplish this too, but we're not sure it will give
        us the performance we need. This is a first step, and it accomplishes
        most of the value of using Weak<T>.

        * heap/Heap.cpp:
        (JSC::Heap::destroy):
        (JSC::Heap::markRoots):
        (JSC::Heap::reset): Finalize small strings just like other weak handles.

        * runtime/SmallStrings.cpp:
        (JSC::finalize):
        (JSC::SmallStrings::finalizeSmallStrings):
        * runtime/SmallStrings.h: Make all small strings trivially weak, instead
        of having an "all for one, one for all" memory model.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87448 268f45cc-cd09-0410-ab3c-d52691b4dbfc