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..094dd755c99f952140a4f4a22dbaac6fdd75cbb6 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -744,6 +744,10 @@ IPC_STRUCT_END()
 IPC_MESSAGE_CONTROL1(ViewMsg_SetNextPageID,
                      int32 /* next_page_id */)
 
+// Used to fix page_id offsets when pruning history.

[brettw, 2011/08/09 18:14:40]

Can you expand on this? It doesn't actually tell what it means. I'm actually a
bit confused. I was trying to write a comment for my code sample below and I was
having trouble describing what this message is telling us to do.

With a name like offset, I would expect things to be shifted. Is my
understanding of what this is supposed to do correct? : It sets sets the current
page as the most recent one in history with the given number of previous history
items. And we assume all previous history items are in other processes since
this is a preloaded page with no history.

If that's true, "OffsetAndPrune" seems maybe misleading or insufficient. Maybe
this is complicated enough that we just need to name this as prerendering
related? :
  ViewMsg_SetHistoryStateForPreloadedPage, history_length)

[cbentzel, 2011/08/09 18:21:14]

This impacts Instant as well [and yes, names are confusing].

Your assumption is correct. Basically, when the prerendered page is created it
will typically have a history_page_ids_ of

[4]

If there were 5 previous history items at the time OffsetAndPruneHistory is
called, it will look like

[-1, -1, -1, -1, -1, 4]

Perhaps I should call this "ShiftHistory"?

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

This would be a great example to put in the comment here, and maybe the handler
in render_view.cc should reference this location.

Maybe something like: SetHistoryLengthAndClear
and call the parameter "length"?

+IPC_MESSAGE_ROUTED1(ViewMsg_OffsetAndPruneHistory,
+                    int32 /* offset */)
+
 // 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