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

chrome/common/extensions/extension_messages.h

 // Copyright (c) 2011 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.
 
 // IPC messages for extensions.
 // Multiply-included message file, hence no include guard.
 
 #include "base/shared_memory.h"
 #include "base/values.h"
 #include "chrome/common/extensions/extension.h"
@@ skipping 255 matching lines @@
 // event.
 IPC_MESSAGE_CONTROL2(ExtensionHostMsg_RemoveListener,
                      std::string /* extension_id */,
                      std::string /* name */)
 
 // Notify the browser that the extension is idle so it's lazy background page
 // can be closed.
 IPC_MESSAGE_CONTROL1(ExtensionHostMsg_ExtensionIdle,
                      std::string /* extension_id */)
 
+// Notify the browser that an event has finished being dispatched.
+IPC_MESSAGE_CONTROL1(ExtensionHostMsg_ExtensionEventAck,
+                     std::string /* extension_id */)
+
+
 // Open a channel to all listening contexts owned by the extension with
 // the given ID.  This always returns a valid port ID which can be used for
 // sending messages.  If an error occurred, the opener will be notified
 // asynchronously.
 IPC_SYNC_MESSAGE_CONTROL4_1(ExtensionHostMsg_OpenChannelToExtension,
                             int /* routing_id */,
                             std::string /* source_extension_id */,
                             std::string /* target_extension_id */,
                             std::string /* channel_name */,
                             int /* port_id */)
@@ skipping 60 matching lines @@
 // Response to the renderer for the above message.
 IPC_MESSAGE_ROUTED3(ExtensionMsg_GetAppNotifyChannelResponse,
                     std::string /* channel_id */,
                     std::string /* error */,
                     int32 /* callback_id */)
 
 // Deliver a message sent with ExtensionHostMsg_PostMessage.
 IPC_MESSAGE_ROUTED2(ExtensionMsg_DeliverMessage,
                     int /* target_port_id */,
                     std::string /* message */)