Fixed a recently-introduced assertion failure when handling 404 errors.

        * WebView/WebDataSource.m:
        (-[WebDataSource _handleFallbackContent]):
        Use the -[WebFrame _bridge] instead of -[WebDataSource _bridge].  The former is not valid until the data
        source has been committed, as is the case when the resource fails to load.  The latter is safe to call at
        any time.  This broke last night with Maciej's change to WebFrameResourceLoader.  The old code used to call
        -[WebFrame _bridge].



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@14843 268f45cc-cd09-0410-ab3c-d52691b4dbfc