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

content/renderer/render_view.cc

Index: content/renderer/render_view.cc
diff --git a/content/renderer/render_view.cc b/content/renderer/render_view.cc
index c74a3cd55f7b037f6ded3ff0cca7b3c944dfcc13..dd31fb7e922778713effed968a234cb92882269d 100644
--- a/content/renderer/render_view.cc
+++ b/content/renderer/render_view.cc
@@ -704,6 +704,8 @@ bool RenderView::OnMessageReceived(const IPC::Message& message) {
 #endif
     IPC_MESSAGE_HANDLER(ViewMsg_UpdateRemoteAccessClientFirewallTraversal,
                         OnUpdateRemoteAccessClientFirewallTraversal)
+    IPC_MESSAGE_HANDLER(ViewMsg_SetHistoryLengthAndClear,
+                        OnSetHistoryLengthAndClear)
     // Have the super handle all other messages.
     IPC_MESSAGE_UNHANDLED(handled = RenderWidget::OnMessageReceived(message))
   IPC_END_MESSAGE_MAP()
@@ -1018,6 +1020,32 @@ void RenderView::OnScrollFocusedEditableNodeIntoView() {
   }
 }
 
+void RenderView::OnSetHistoryLengthAndClear(int history_length) {
+  DCHECK(history_length >= 0);
+
+  // history_list_length_ may be 0 if this is called between
+  // a navigate and a commit of the provisional load. Otherwise,
+  // only add one entry, regardless of how long the current history is.
+  // TODO(cbentzel): Investigate what happens if a prerendered page
+  //   navigates to several entries before it is swapped in. Cropping
+  //   those may be a bad idea.
+  int new_history_list_length = history_length;
+  if (history_list_length_ > 0)
+    ++new_history_list_length;
+
+  DCHECK(page_id_ == -1 ||
+         (history_list_offset_ >= 0 &&
+          page_id_ == history_page_ids_[history_list_offset_]));
+
+  // Generate the new list.
+  std::vector<int32> new_history_page_ids(new_history_list_length, -1);
+  if (page_id_ != -1)
+    new_history_page_ids[new_history_list_length - 1] = page_id_;
+  new_history_page_ids.swap(history_page_ids_);
+  history_list_offset_ = new_history_list_length - 1;
+  history_list_length_ = new_history_list_length;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 // Tell the embedding application that the URL of the active page has changed