Reviewed by Oliver Hunt.

        Removed a few more deprecatedAppends, and removed HeapRoot<T>
        https://bugs.webkit.org/show_bug.cgi?id=56422
        
        Added HeapRootMarker, a privileged class for marking direct heap roots
        that are iterated during each garbage collection. This is easier to use
        and more reliable than HeapRoot<T>, so I've removed HeapRoot<T>.

        * debugger/Debugger.cpp:
        (JSC::evaluateInGlobalCallFrame):
        * debugger/DebuggerCallFrame.cpp:
        (JSC::DebuggerCallFrame::evaluate):
        * interpreter/CallFrame.h:
        (JSC::ExecState::exception):
        * jit/JITStubs.cpp:
        (JSC::DEFINE_STUB_FUNCTION):
        * runtime/Completion.cpp:
        (JSC::evaluate): exception is no longer a HeapRoot<T>, so no need to
        call .get() on it.

        * runtime/Heap.cpp:
        (JSC::Heap::markProtectedObjects):
        (JSC::Heap::markTempSortVectors):
        (JSC::Heap::markRoots):
        * runtime/Heap.h: Updated to use HeapRootMarker.

        * runtime/JSCell.h:
        (JSC::JSCell::MarkStack::append): Added private functions for
        HeapRootMarker to use.

        * runtime/JSGlobalData.h: exception is no longer a HeapRoot<T>.

        * runtime/MarkStack.h:
        (JSC::HeapRootMarker::HeapRootMarker):
        (JSC::HeapRootMarker::mark): Added private functions for
        HeapRootMarker to use.

        * runtime/SmallStrings.cpp:
        (JSC::SmallStrings::markChildren): Updated to use HeapRootMarker.

        * runtime/SmallStrings.h:
        (JSC::SmallStrings::emptyString):
        (JSC::SmallStrings::singleCharacterString):
        (JSC::SmallStrings::singleCharacterStrings): Updated to use HeapRootMarker.

        * runtime/WriteBarrier.h: Removed HeapRoot<T>.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@81191 268f45cc-cd09-0410-ab3c-d52691b4dbfc