Reviewed by Dan Bernstein.

        Recover if the plug-in host dies and we try to instantiate another plugin before we get the port death notification
        
        * Plugins/Hosted/NetscapePluginHostManager.h:
        * Plugins/Hosted/NetscapePluginHostManager.mm:
        (WebKit::NetscapePluginHostManager::pluginHostDied):
        Remove the plug-in host from the map.
        
        (WebKit::NetscapePluginHostManager::instantiatePlugin):
        NetscapePluginHostProxy is no longer refcounted.

        * Plugins/Hosted/NetscapePluginHostProxy.h:
        This is no longer refcounted. Add a set of plug-in instances (unused for now).
        
        * Plugins/Hosted/NetscapePluginHostProxy.mm:
        (WebKit::NetscapePluginHostProxy::NetscapePluginHostProxy):
        Create a death notification port.
        
        (WebKit::NetscapePluginHostProxy::pluginHostDied):
        Tell the manager that we're gone and delete ourselves.
        
        (WebKit::NetscapePluginHostProxy::deathPortCallback):
        New CFMachPort callback.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@38905 268f45cc-cd09-0410-ab3c-d52691b4dbfc