Reviewed by Xan Lopez.

        [GTK] Optimize foldCase, toLower and toUpper methods in glib unicode backend
        https://bugs.webkit.org/show_bug.cgi?id=48625

        GLib methods use UTF-8 strings, so we have to convert from UTF-16 to
        UTF-8 to perform the case operations and then convert back the result to
        UTF-16. GLib conversion methods return a new allocated string, so we
        have to memcpy the result into the destination buffer too. Using our
        own methods to convert between UTF-8 and UTF-16 from wtf/unicode/UTF8.h
        we don't need such memcpy, since they take an already allocated buffer
        rather than returning a new one. There's another optimization for the
        case when the destination buffer is not large enough. In that case,
        methods should return the expected destination buffer size and are
        called again with a new buffer. We can avoid the conversion to UTF-16 by
        pre-calculating the required size for the destination buffer.

        * wtf/unicode/glib/UnicodeGLib.cpp:
        (WTF::Unicode::getUTF16LengthFromUTF8):
        (WTF::Unicode::convertCase):
        (WTF::Unicode::foldCase):
        (WTF::Unicode::toLower):
        (WTF::Unicode::toUpper):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@72662 268f45cc-cd09-0410-ab3c-d52691b4dbfc