Reviewed by Adam, Darin, and Geoff.

        Fix for <rdar://problem/5000470> REGRESSION: The IM reconvert
        function returns incorrect symbol due to inconsistent range
        domains in TSM

        Text Services Management uses ranges provided by the
        NSTextInput API to index into the string return by
        -[WebHTMLView string]. As a result some input methods
        incorrectly get their candidate text from the beginning
        of the document instead of from the input element.

        TSM prefers to query -textStorage over -string so as a
        workaround we provide an implementation of -textStorage that
        returns the content of the current text input.  TSM only ever
        queries the result of textStorage as an NSAttributedString so
        we do not need to implement a fake NSTextStorage class

        This should not cause harm to anything else as textStorage is
        actually a method on NSTextView, which we clearly are not. TSM
        only queries the method because it uses respondsToSelector to
        control behaviour.

        * WebView/WebHTMLView.mm:
        (-[WebHTMLView textStorage]):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@21728 268f45cc-cd09-0410-ab3c-d52691b4dbfc