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/common/HostChannel.cpp b/plugins/common/HostChannel.cpp
index 90552a4..bff1b21 100644
--- a/plugins/common/HostChannel.cpp
+++ b/plugins/common/HostChannel.cpp
@@ -68,6 +68,7 @@
 
 bool HostChannel::init(SessionHandler* handler, int minProtoVers,
     int maxProtoVers, const std::string& hostedHtmlVers) {
+  this->handler = handler;
   Debug::log(Debug::Debugging)
       << "  negotiating versions - we support protocol " << minProtoVers
       << " through " << maxProtoVers << ", hostedHtmlVersion=" << hostedHtmlVers