We need to treat plug-in streams like subresources except they don't change the loading state of the data source.

        * Plugins.subproj/WebNetscapePluginStream.m:
        (-[WebNetscapePluginStream start]): call _addPluginStream
        (-[WebNetscapePluginStream cancel]): call _removePluginStream
        (-[WebNetscapePluginStream handleDidFinishLoading:]): call _removePluginStream
        (-[WebNetscapePluginStream handle:didFailLoadingWithError:]): call _removePluginStream
        * WebView.subproj/WebDataSourcePrivate.h:
        * WebView.subproj/WebDataSourcePrivate.m:
        (-[WebDataSourcePrivate dealloc]): release pluginStreams
        (-[WebDataSource _addPluginStream:]): just like addSubresourceClient except don't change the loading state.
        (-[WebDataSource _removePluginStream:]): just like removeSubresourceClient except don't change the loading state.
        (-[WebDataSource _stopLoading]): stop plug-ins streams
        (-[WebDataSource _makeHandleDelegates:deferCallbacks:]):
        (-[WebDataSource _defersCallbacksChanged]): call _makeHandleDelegates:deferCallbacks: for subresources and plug-in streams.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@2737 268f45cc-cd09-0410-ab3c-d52691b4dbfc