Have LazyBackgroundTaskQueue notify consumers when the transient page fails to

chrome/browser/extensions/extension_message_service.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/extension_message_service.h"
 
 #include "base/atomic_sequence_num.h"
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/json/json_writer.h"
@@ skipping 424 matching lines @@
     }
   }
 
   return false;
 }
 
 void ExtensionMessageService::PendingOpenChannel(
     const OpenChannelParams& params_in,
     int source_process_id,
     ExtensionHost* host) {
+  if (!host)
+    return;  // TODO(mpcomplete): notify source of disconnect?
+
   // Re-lookup the source process since it may no longer be valid.
   OpenChannelParams params = params_in;
   params.source = content::RenderProcessHost::FromID(source_process_id);
   if (!params.source)
     return;
 
   params.receiver = MessagePort(host->render_process_host(),
                                 MSG_ROUTING_CONTROL);
   OpenChannelImpl(params);
 }