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..fb17fa9c74c20a7cb6b3b57b6f0da428639f6ea1 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -744,6 +744,14 @@ IPC_STRUCT_END()
 IPC_MESSAGE_CONTROL1(ViewMsg_SetNextPageID,
                      int32 /* next_page_id */)
 
+// Shifts the history of page_id's for a page by |history_length| entries.
+// The main use for this is prerendered pages, but Instant pages also use this.
+// When a prerendered page is created, it has a history of only one page_id.
+// When the prerendered page is swapped in, this page_id needs to be shifted
+// over by the current history length.
+IPC_MESSAGE_ROUTED1(ViewMsg_ShiftHistory,

[brettw, 2011/08/09 19:38:34]

I still don't feel like this is shifting history. This only "shifts" if the
current entry is the first one. If you have 3 current history items, this
doesn't shift at all. This is what I was trying to communicate with my previous
suggestions.

Also: I really liked your example of [-1, -1, -1, 4] for an example length of 3.
Can you add that here?

+                    int /* history_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