Keep a copy of page id in RenderViewHost.

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 562 matching lines @@
 
 // Notifies that the initial empty document of a view has been accessed.
 // After this, it is no longer safe to show a pending navigation's URL without
 // making a URL spoof possible.
 IPC_MESSAGE_ROUTED0(FrameHostMsg_DidAccessInitialDocument)
 
 // Sent when the frame sets its opener to null, disowning it for the lifetime of
 // the window. Sent for top-level frames.
 IPC_MESSAGE_ROUTED0(FrameHostMsg_DidDisownOpener)
 
+// Notifies the browser that a page id was assigned.
+IPC_MESSAGE_ROUTED1(FrameHostMsg_DidAssignPageId,
+                    int32 /* page_id */)
+
 // Changes the title for the page in the UI when the page is navigated or the
 // title changes. Sent for top-level frames.
 IPC_MESSAGE_ROUTED3(FrameHostMsg_UpdateTitle,
                     int32 /* page_id */,
                     base::string16 /* title */,
                     blink::WebTextDirection /* title direction */)
 
 // Change the encoding name of the page in UI when the page has detected
 // proper encoding name. Sent for top-level frames.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_UpdateEncoding,
@@ skipping 176 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