IE, Chrome, Firefox plugins: gracefully disconnect when server connection drops.

1) Plugins fails gracefully in the face of a disconnect, returning undefined instead of making lots of noise.

2) Plugins invokes hosted.html's __gwt_disconnected() method the first time the session is detected as dropped.  This glasspanels the app.

Review by: jat

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7129 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/plugins/xpcom/FFSessionHandler.h b/plugins/xpcom/FFSessionHandler.h
index f7fec2b..1da4288 100755
--- a/plugins/xpcom/FFSessionHandler.h
+++ b/plugins/xpcom/FFSessionHandler.h
@@ -41,6 +41,7 @@
   void disconnect();
 
 protected:
+  virtual void disconnectDetectedImpl();
   virtual void freeValue(HostChannel& channel, int idCount, const int* ids);
   virtual void loadJsni(HostChannel& channel, const std::string& js);
   virtual bool invoke(HostChannel& channel, const Value& thisObj, const std::string& methodName,