Makes instant run before unload listeners.

chrome/browser/tab_contents/navigation_controller.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/tab_contents/navigation_controller.h"
 
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/string_util.h"
 #include "base/time.h"
@@ skipping 961 matching lines @@
     // There's a transient entry. In this case we want the last committed to
     // point to the previous entry.
     transient_entry_index_ = entry_count() - 1;
     if (last_committed_entry_index_ != -1)
       last_committed_entry_index_--;
   }
 
   // Take over the session id from source.
   session_id_ = source->session_id_;
 
-  // Reset source's session id as we're taking it over.
-  source->session_id_.clear();
+  // Reset source's session id as we're taking it over. We give it a new id in
+  // case source is added later on, which can happen with instant enabled if the
+  // tab has a before unload handler.
+  source->session_id_ = SessionID();
 }
 
 void NavigationController::PruneAllButActive() {
   int prune_count = entry_count();
   if (transient_entry_index_ != -1) {
     // There is a transient entry. Prune up to it.
     DCHECK_EQ(entry_count() - 1, transient_entry_index_);
     prune_count = transient_entry_index_;
     transient_entry_index_ = 0;
     last_committed_entry_index_ = -1;
@@ skipping 205 matching lines @@
   size_t insert_index = 0;
   for (int i = 0; i < max_index; i++) {
     // When cloning a tab, copy all entries except interstitial pages
     if (source.entries_[i].get()->page_type() != INTERSTITIAL_PAGE) {
       entries_.insert(entries_.begin() + insert_index++,
                       linked_ptr<NavigationEntry>(
                           new NavigationEntry(*source.entries_[i])));
     }
   }
 }