Closing a window requires closing web contents now.

In this way, we can prevent the close of window by using beforeunload
handler.

browser/native_window.h

@@ -90,9 +90,18 @@ class NativeWindow : public content::WebContentsDelegate,
   virtual void FlashFrame(bool flash) = 0;
   virtual void SetKiosk(bool kiosk) = 0;
   virtual bool IsKiosk() = 0;
+
   virtual void ShowDevTools();
   virtual void CloseDevTools();
 
+  // Close the web page in this window and then desctruct.
+  virtual void RequestToDestroyWindow();
+
+  // Used by platform dependent code to determine whether the window can be
+  // closed. A window can only be closed when the beforeunload handler
+  // doesn't prevent it.
+  bool CanClose();
+
   content::WebContents* GetWebContents() const;
 
   void AddObserver(NativeWindowObserver* obs) {
@@ -119,6 +128,10 @@ class NativeWindow : public content::WebContentsDelegate,
                                   content::WebContents* new_contents) OVERRIDE;
   virtual content::JavaScriptDialogManager*
       GetJavaScriptDialogManager() OVERRIDE;
+  virtual void CloseContents(content::WebContents* source) OVERRIDE;
+  virtual void BeforeUnloadFired(content::WebContents* source,
+                                 bool proceed,
+                                 bool* proceed_to_fire_unload) OVERRIDE;
 
   // Implementations of content::WebContentsObserver.
   virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;
@@ -149,6 +162,9 @@ class NativeWindow : public content::WebContentsDelegate,
 
   scoped_ptr<brightray::InspectableWebContents> inspectable_web_contents_;
 
+  bool window_going_to_destroy_;
+  bool can_destroy_window_;
+
   DISALLOW_COPY_AND_ASSIGN(NativeWindow);
 };