ChromeFrame tabs would hang at times while closing. This would randomly occur...

chrome/test/automation/automation_messages_internal.h

 // Copyright (c) 2010 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.
 
 // Defines the IPC messages used by the automation interface.
 
 // This header is meant to be included in multiple passes, hence no traditional
 // header guard.
 // See ipc_message_macros.h for explanation of the macros and passes.
 
@@ skipping 1386 matching lines @@
                              int, bool)
 
   // This message requests the external tab identified by the tab handle
   // passed in be closed.
   // Request:
   //   -int: Tab handle
   // Response:
   //   None expected
   IPC_MESSAGE_ROUTED1(AutomationMsg_CloseExternalTab, int)
 
+  // This message requests that the external tab identified by the tab handle
+  // runs unload handlers if any on the current page.
+  // Request:
+  //   -int: Tab handle
+  //   -gfx::NativeWindow: notification window
+  //   -int: notification message.
+  // Response:
+  //   None expected
+  IPC_MESSAGE_ROUTED3(AutomationMsg_RunUnloadHandlers, int, gfx::NativeWindow,
+                      int)
+
 IPC_END_MESSAGES(Automation)