Fix regression with back-button not working on prerendered and instant pages.

content/common/view_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 page rendering.
 // Multiply-included message file, hence no include guard.
 
 #include "base/process.h"
 #include "base/shared_memory.h"
 #include "content/common/common_param_traits.h"
@@ skipping 726 matching lines @@
   IPC_STRUCT_MEMBER(string16, frame_name)
 IPC_STRUCT_END()
 
 // Messages sent from the browser to the renderer.
 
 // Used typically when recovering from a crash.  The new rendering process
 // sets its global "next page id" counter to the given value.
 IPC_MESSAGE_CONTROL1(ViewMsg_SetNextPageID,
                      int32 /* next_page_id */)
 
+// Sets the history length of page_ids for a RenderView to
+// |length| entries, and moves the current page_id to the last
+// entry if it is valid.
+// The main use for this is prerendered pages, but Instant pages also use this.
+// For example, assume that there are 3 entries in the history when a
+// prerendered page is created. The new prerendered page will have a single
+// entry history like [7]. When it is swapped in, we need to extend the history
+// so it has a total length of 4 (3 for the previous history, 1 for the
+// prerendered page), so it looks like [-1 -1 -1 7].
+IPC_MESSAGE_ROUTED1(ViewMsg_SetHistoryLengthAndClear,
+                    int /* length */)
+
 // Sends System Colors corresponding to a set of CSS color keywords
 // down the pipe.
 // This message must be sent to the renderer immediately on launch
 // before creating any new views.
 // The message can also be sent during a renderer's lifetime if system colors
 // are updated.
 // TODO(jeremy): Possibly change IPC format once we have this all hooked up.
 IPC_MESSAGE_ROUTED1(ViewMsg_SetCSSColors,
                     std::vector<CSSColors::CSSColorMapping>)
 
@@ skipping 1283 matching lines @@
                     int32 /* complete status */)
 
 // Request updated information about the client firewall traversal policy.
 // Will result in a ViewMsg_UpdateRemoteAccessClientFirewallTraversal message
 // being sent back.
 IPC_MESSAGE_ROUTED0(ViewHostMsg_RequestRemoteAccessClientFirewallTraversal)
 
 // Notifies the browser of an event occurring in the media pipeline.
 IPC_MESSAGE_CONTROL1(ViewHostMsg_MediaLogEvent,
                      media::MediaLogEvent /* event */)