Reviewed by Oliver Hunt.

        Removed more cases of DeprecatedPtr (exception, SmallStrings)
        https://bugs.webkit.org/show_bug.cgi?id=56332

        * runtime/Identifier.cpp:
        (JSC::Identifier::add):
        (JSC::Identifier::addSlowCase): Use a variable instead of a hard-coded
        constant, to make this code less brittle.

        * runtime/JSGlobalData.h: Use HeapRoot instead of DeprecatedPtr because
        this reference is owned and managed directly by the heap.

        * runtime/JSString.cpp:
        (JSC::JSString::substringFromRope):
        * runtime/JSString.h:
        (JSC::jsSingleCharacterString):
        (JSC::jsSingleCharacterSubstring):
        (JSC::jsString):
        (JSC::jsStringWithFinalizer):
        (JSC::jsSubstring):
        (JSC::jsOwnedString): Use a variable instead of a hard-coded
        constant, to make this code less brittle.

        * runtime/SmallStrings.cpp:
        (JSC::SmallStringsStorage::rep):
        (JSC::SmallStringsStorage::SmallStringsStorage):
        (JSC::SmallStrings::SmallStrings):
        (JSC::SmallStrings::markChildren):
        (JSC::SmallStrings::clear):
        (JSC::SmallStrings::count): Use a variable instead of a hard-coded
        constant, to make this code less brittle.

        * runtime/SmallStrings.h:
        (JSC::SmallStrings::singleCharacterString): Use HeapRoot instead of
        DeprecatedPtr because these references are owned and managed directly by
        the heap.
        
        Stop using FixedArray because we only want a very limited set
        of classes to be able to use HeapRoot. (Replaced with manual ASSERTs.)

        * runtime/WriteBarrier.h:
        (JSC::operator==):
        (JSC::WriteBarrier::WriteBarrier):
        (JSC::HeapRoot::HeapRoot):
        (JSC::HeapRoot::operator=): Added HeapRoot, which is allowed to set
        without write barrier because we assume all HeapRoots are scanned during
        all GC passes.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@81071 268f45cc-cd09-0410-ab3c-d52691b4dbfc