Gracefully handle nested frame detaches.

Source/core/loader/FrameLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008 Alp Toker <alp@atoker.com>
  * Copyright (C) Research In Motion Limited 2009. All rights reserved.
  * Copyright (C) 2011 Kris Jordan <krisjordan@gmail.com>
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ skipping 1047 matching lines @@
     // The caller must protect a reference to m_frame.
     ASSERT(m_frame->refCount() > 1);
 #endif
     if (m_documentLoader)
         m_documentLoader->detachFromFrame();
     m_documentLoader = nullptr;
 
     Frame* parent = m_frame->tree().parent();
     if (parent && parent->isLocalFrame())
         toLocalFrame(parent)->loader().scheduleCheckCompleted();
-    m_progressTracker->dispose();
-    m_progressTracker.clear();
+    if (m_progressTracker) {
+        m_progressTracker->dispose();
+        m_progressTracker.clear();
+    }
     setOpener(0);
 }
 
 void FrameLoader::receivedMainResourceError(DocumentLoader* loader, const ResourceError& error)
 {
     // Retain because the stop may release the last reference to it.
     RefPtrWillBeRawPtr<LocalFrame> protect(m_frame.get());
     RefPtrWillBeRawPtr<DocumentLoader> protectDocumentLoader(loader);
 
     // FIXME: We really ought to be able to just check for isCancellation() here, but there are some
@@ skipping 331 matching lines @@
 
     // FIXME: We need a way to propagate insecure content policy flags to
     // out-of-process frames. For now, we'll always use default behavior.
     if (!parentFrame->isLocalFrame())
         return SecurityContext::InsecureContentDoNotUpgrade;
 
     return toLocalFrame(parentFrame)->document()->insecureContentPolicy();
 }
 
 } // namespace blink