Don't close background pages if there is a pending response (callback).

chrome/browser/extensions/extension_event_router.h

Index: chrome/browser/extensions/extension_event_router.h
diff --git a/chrome/browser/extensions/extension_event_router.h b/chrome/browser/extensions/extension_event_router.h
index 55831ed7a1d35effb1aff643903fe0c1ec8d20e8..ade675ea58cdf7c80c84fd4545ac68185c9f6069 100644
--- a/chrome/browser/extensions/extension_event_router.h
+++ b/chrome/browser/extensions/extension_event_router.h
@@ -88,6 +88,13 @@ class ExtensionEventRouter : public content::NotificationObserver {
       const std::string& cross_incognito_args,
       const GURL& event_url);
 
+  // Record the Event Ack from the renderer. (One less event in-flight.)
+  void OnExtensionEventAck(const std::string& extension_id);
+
+  // Check if there are any Extension Events that have not yet been acked by
+  // the renderer.
+  bool HasInFlightEvents(const std::string& extension_id);
+
  protected:
   // The details of an event to be dispatched.
   struct ExtensionEvent;
@@ -132,6 +139,11 @@ class ExtensionEventRouter : public content::NotificationObserver {
                    linked_ptr<PendingEventsList> > PendingEventsPerExtMap;
   PendingEventsPerExtMap pending_events_;
 
+  // Track of the number of dispatched events that have not yet sent an
+  // ACK from the renderer.
+  void IncrementInFlightEvents(const std::string& extension_id);
+  std::map<std::string, int> in_flight_events_;
+
   DISALLOW_COPY_AND_ASSIGN(ExtensionEventRouter);
 };