Reviewed by Kevin Decker.

        Refactor some duplicate plug-in clean up code into shared functions.

        * Plugins/WebPluginController.mm:
        (-[WebPluginController stopOnePlugin:]): New, stop a plug-in.
        (-[WebPluginController destroyOnePlugin:]): New, destroy plug-in.
        (-[WebPluginController stopAllPlugins]): Call stopOnePlugin.
        (-[WebPluginController destroyPlugin:]): Call stopOnePlugin and destroyOnePlugin.
        (-[WebPluginController destroyAllPlugins]): Call destroyOnePlugin.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@50305 268f45cc-cd09-0410-ab3c-d52691b4dbfc