Reviewed by Adele.

    Reviewed by Adele.

        - fix <rdar://problem/5383104> REGRESSION: XHR.responseText is null instead of empty string
          in http/tests/xmlhttprequest/zero-length-response.html

        The new code to handle out of memory conditions was turning a "" into a null string.

        * kjs/ustring.h: Removed UCharReference, which has long been obsolete and unused.
        Removed copyForWriting, which was only used for the upper/lowercasing code and for
        UCharReference.
        * kjs/ustring.cpp:
        (KJS::allocChars): Removed special case that made this fail (return 0) when passed 0.
        Instead assert that we're not passed 0. Also added an overflow check for two reasons:
        1) for sizes that aren't checked this prevents us from allocating a buffer that's too
        small, and 2) for sizes where we overflowed in the expandedSize function and returned
        overflowIndicator, it guarantees we fail.
        (KJS::reallocChars): Ditto.
        (KJS::UString::expandedSize): Return a large number, overflowIndicator, rather than 0
        for cases where we overflow.
        (KJS::UString::spliceSubstringsWithSeparators): Added a special case for empty string so
        we don't call allocChars with a length of 0.
        (KJS::UString::operator=): Added special characters for both 0 and empty string so we
        match the behavior of the constructor. This avoids calling allocChars with a length of 0
        and making a null string rather than an empty string in that case, and also matches the
        pattern used in the rest of the functions.
        (KJS::UString::operator[]): Made the return value const so code that tries to use the
        operator to modify the string will fail.

        * kjs/string_object.cpp: (KJS::StringProtoFunc::callAsFunction): Rewrote uppercasing and
        lowercasing functions so they don't need copyForWriting any more -- it wasn't really doing
        any good for optimization purposes. Instead use a Vector and releaseBuffer.

        * wtf/unicode/icu/UnicodeIcu.h: Eliminate one of the versions of toLower/toUpper -- we now
        only need the version where both a source and destination buffer is passed in, not the one
        that works in place.
        * wtf/unicode/qt4/UnicodeQt4.h: Ditto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@24919 268f45cc-cd09-0410-ab3c-d52691b4dbfc