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

content/browser/tab_contents/navigation_controller.cc

Index: content/browser/tab_contents/navigation_controller.cc
diff --git a/content/browser/tab_contents/navigation_controller.cc b/content/browser/tab_contents/navigation_controller.cc
index e29ecc31ded29d91e7b56219710a56d94a821cfe..0144fef9b691ea047279e75249cd6688102e25f0 100644
--- a/content/browser/tab_contents/navigation_controller.cc
+++ b/content/browser/tab_contents/navigation_controller.cc
@@ -931,6 +931,9 @@ void NavigationController::CopyStateFromAndPrune(NavigationController* source,
     if (last_committed_entry_index_ != -1)
       last_committed_entry_index_--;
   }
+
+  // Update the history in the RenderView.
+  tab_contents_->SetHistoryLengthAndClear(max_source_index);
 }
 
 void NavigationController::PruneAllButActive() {