Prerendered pages are swapped in at browser::Navigate time.

chrome/common/prerender_messages.h

Index: chrome/common/prerender_messages.h
diff --git a/chrome/common/prerender_messages.h b/chrome/common/prerender_messages.h
new file mode 100644
index 0000000000000000000000000000000000000000..4c965a94f9fdeae3a7abb37734a4d1fcf63bd7a0
--- /dev/null
+++ b/chrome/common/prerender_messages.h
@@ -0,0 +1,25 @@
+// Copyright (c) 2012 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.
+
+// Multiply-included message file, no traditional include guard.
+#include "googleurl/src/gurl.h"
+#include "ipc/ipc_message.h"
+#include "ipc/ipc_message_macros.h"
+#include "ipc/ipc_param_traits.h"
+
+#define IPC_MESSAGE_START PrerenderMsgStart
+
+// Tells a renderer if it's currently being prerendered.  Must only be set
+// to true before any navigation occurs, and only set to false at most once
+// after that.
+IPC_MESSAGE_ROUTED1(PrerenderMsg_SetIsPrerendering,

[Charlie Reis, 2012/03/01 19:06:03]

nit: SetAllowPrerendering, perhaps?  Right now it sounds like we're actively
prerendering a page, but we use AddPrerenderURL for that.

[cbentzel, 2012/03/05 15:38:58]

This is sent to the actual prerendered RenderView to let it know that it is
prerendering.

AddPrerenderURL and RemovePrerenderURL are sent to the RenderView process that
created the prerender to let it know about possible ones. 

+                    bool /* whether the RenderView is prerendering */)
+
+// Specifies that a URL is currently being prerendered.
+IPC_MESSAGE_CONTROL1(PrerenderMsg_AddPrerenderURL,
+                     GURL /* url */)
+
+// Specifies that a URL is no longer being prerendered.
+IPC_MESSAGE_CONTROL1(PrerenderMsg_RemovePrerenderURL,
+                     GURL /* url */)