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

content/common/view_messages.h

Index: content/common/view_messages.h
diff --git a/content/common/view_messages.h b/content/common/view_messages.h
index eaf0655fc4ceb03683b95cd42a5d974b1c53c33c..ab3372142ce467283441c490c67f1c88654092a3 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -744,6 +744,18 @@ IPC_STRUCT_END()
 IPC_MESSAGE_CONTROL1(ViewMsg_SetNextPageID,
                      int32 /* next_page_id */)
 
+// Set's the history length of page_id's for a RenderView by

[Charlie Reis, 2011/08/09 20:59:31]

Nit: No apostrophes in either case, and s/by/to/.  Maybe "Sets the length of
history_page_ids_"?

+// |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].

[Charlie Reis, 2011/08/09 20:59:31]

Please mention the invariant about when this is safe to send.  It must be sent
before the new renderer has a chance to navigate back or forward, correct?

+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