Makes instant run before unload listeners.

chrome/browser/tab_contents/navigation_controller.cc

Index: chrome/browser/tab_contents/navigation_controller.cc
diff --git a/chrome/browser/tab_contents/navigation_controller.cc b/chrome/browser/tab_contents/navigation_controller.cc
index af5d3b7bc7ee40767bee32ce2d4b294ea535cdf3..e4c5aaca603406240d25f3e277601798fd94f683 100644
--- a/chrome/browser/tab_contents/navigation_controller.cc
+++ b/chrome/browser/tab_contents/navigation_controller.cc
@@ -979,8 +979,10 @@ void NavigationController::CopyStateFromAndPrune(NavigationController* source) {
   // 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() {