Add Browser class to controll when the application should quit.

browser/window_list_observer.h

@@ -17,6 +17,9 @@ class WindowListObserver {
   // Called immediately after a window is removed from the list.
   virtual void OnWindowRemoved(NativeWindow* window) {}
 
+  // Called when a window close is cancelled by beforeunload handler.
+  virtual void OnWindowCloseCancelled(NativeWindow* window) {}
+
   // Called immediately after all windows are closed.
   virtual void OnWindowAllClosed() {}