- fix assertion failure on layout tests by stopping plugins from loading at a clearly defined time
        - add more assertions for safety

        * WebView/WebDataSource.m:
        (-[WebDataSourcePrivate dealloc]): Removed obsolete comment.
        (-[WebDataSource _updateLoading]): Add assertion ensuring this method is only called
        at a time when this data source is the one that might be loading for a frame.
        (-[WebDataSource _stopLoading]): Stop loading plugins as a FIXME suggests we should.
        * WebView/WebFrameLoader.m:
        (-[WebFrameLoader isLoadingPlugIns]): New helper method.
        (-[WebFrameLoader isLoading]): Consider plugin loads too - otherwise we won't stop them
        at stopLoading time.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@15756 268f45cc-cd09-0410-ab3c-d52691b4dbfc