Avoid stale navigation requests without excessive page id knowledge in the renderer process.

content/common/frame_messages.h

 // Copyright 2013 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 interacting with frames.
 // Multiply-included message file, hence no include guard.
 
 #include "content/common/content_export.h"
 #include "content/common/content_param_traits.h"
 #include "content/common/frame_message_enums.h"
@@ skipping 216 matching lines @@
   IPC_STRUCT_MEMBER(content::CommonNavigationParams, common_params)
   IPC_STRUCT_MEMBER(content::RequestNavigationParams, request_params)
   IPC_STRUCT_MEMBER(content::CommitNavigationParams, commit_params)
 
   // The page_id for this navigation, or -1 if it is a new navigation.  Back,
   // Forward, and Reload navigations should have a valid page_id.  If the load
   // succeeds, then this page_id will be reflected in the resultant
   // FrameHostMsg_DidCommitProvisionalLoad message.
   IPC_STRUCT_MEMBER(int32, page_id)
 
-  // If page_id is -1, then pending_history_list_offset will also be -1.
-  // Otherwise, it contains the offset into the history list corresponding to
-  // the current navigation.
+  // For history navigations, this is the offset in the history list of the
+  // pending load. For non-history navigations, this will be ignored.
   IPC_STRUCT_MEMBER(int, pending_history_list_offset)
 
   // Informs the RenderView of where its current page contents reside in
   // session history and the total size of the session history list.
   IPC_STRUCT_MEMBER(int, current_history_list_offset)
   IPC_STRUCT_MEMBER(int, current_history_list_length)
 
   // Informs the RenderView the session history should be cleared. In that
   // case, the RenderView needs to notify the browser that the clearing was
   // succesful when the navigation commits.
@@ skipping 557 matching lines @@
 IPC_MESSAGE_ROUTED0(FrameHostMsg_DidFirstVisuallyNonEmptyPaint)
 
 #if defined(OS_MACOSX) || defined(OS_ANDROID)
 
 // Message to show/hide a popup menu using native controls.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_ShowPopup,
                     FrameHostMsg_ShowPopup_Params)
 IPC_MESSAGE_ROUTED0(FrameHostMsg_HidePopup)
 
 #endif