Fix null pointer dereference in ChromeClientImpl::openBeforeUnloadConfirmPanelDelegate.

Source/core/page/ChromeClient.cpp

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2012, Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 94 matching lines @@
     // otherwise cause the load to continue while we're in the middle of
     // executing JavaScript.
     ScopedPageLoadDeferrer deferrer;
 
     ScopedJavaScriptDialogInstrumentation instrumentation(frame, message);
     return (chromeClient->*function)(&frame, message, parameters...);
 }
 
 bool ChromeClient::openBeforeUnloadConfirmPanel(const String& message, LocalFrame* frame)
 {
+    ASSERT(frame);
     return openJavaScriptDialog(this, &ChromeClient::openBeforeUnloadConfirmPanelDelegate, *frame, message);
 }
 
 void ChromeClient::openJavaScriptAlert(LocalFrame* frame, const String& message)
 {
     ASSERT(frame);
     if (!canOpenModalIfDuringPageDismissal(frame->tree().top(), ChromeClient::AlertDialog, message))
         return;
     openJavaScriptDialog(this, &ChromeClient::openJavaScriptAlertDelegate, *frame, message);
 }
@@ skipping 60 matching lines @@
 {
     // Defer loads in case the client method runs a new event loop that would
     // otherwise cause the load to continue while we're in the middle of
     // executing JavaScript.
     ScopedPageLoadDeferrer deferrer;
 
     printDelegate(frame);
 }
 
 } // namespace blink