Move the event attach/detach logic on unload from event.js to

extensions/renderer/dispatcher.cc

Index: extensions/renderer/dispatcher.cc
diff --git a/extensions/renderer/dispatcher.cc b/extensions/renderer/dispatcher.cc
index c61b18897e8f8a1156f08a285d020203422925c4..46abc4326c6cefaefb56e1398dd6e9c882a46295 100644
--- a/extensions/renderer/dispatcher.cc
+++ b/extensions/renderer/dispatcher.cc
@@ -365,7 +365,8 @@ void Dispatcher::WillReleaseScriptContext(
     return;
 
   context->DispatchOnUnloadEvent();
-  // TODO(kalman): add an invalidation observer interface to ScriptContext.
+  // TODO(kalman): Make |request_sender| use |context->AddInvalidationObserver|.
+  // In fact |request_sender_| should really be owned by ScriptContext.
   request_sender_->InvalidateSource(context);
 
   script_context_set_.Remove(context);