Ignore attempts to navigate once a provisional commit has gotten too far along.

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 945 matching lines @@
     RefPtrWillBeRawPtr<LocalFrame> protect(m_frame.get());
 
     // Check if the destination page is allowed to access the previous page's timing information.
     if (m_frame->document()) {
         RefPtr<SecurityOrigin> securityOrigin = SecurityOrigin::create(pdl->request().url());
         pdl->timing().setHasSameOriginAsPreviousDocument(securityOrigin->canRequest(m_frame->document()->url()));
     }
 
     if (m_documentLoader) {
         client()->dispatchWillClose();
         dispatchUnloadEvent();

[michaeln, 2015/06/03 19:43:59]

Maybe a separate issue, but An earlier (pdl != m_provisionalDocumentLoader)
check should probably go here. Otherwise a hashnav from within an onunload
handler looks iffy. I imagine the detachChildren call below will 'break' the
current document and if a hashnav leaves the current doc up... the early return
on 973 would leave you with a busted document.

     }
     m_frame->detachChildren();
     // The previous calls to dispatchUnloadEvent() and detachChildren() can
     // execute arbitrary script via things like unload events. If the executed
     // script intiates a new load or causes the current frame to be detached,
     // we need to abandon the current load.
     if (pdl != m_provisionalDocumentLoader)
         return;
-    if (m_documentLoader)
+
+    if (m_documentLoader) {
+        NavigationDisablerForBeforeUnload navigationDisabler;

[michaeln, 2015/06/03 00:51:50]

I have to rename the class still.

         m_documentLoader->detachFromFrame();
+    }
     // detachFromFrame() will abort XHRs that haven't completed, which can
     // trigger event listeners for 'abort'. These event listeners might detach
     // the frame.
     // TODO(dcheng): Investigate if this can be moved above the check that
     // m_provisionalDocumentLoader hasn't changed.
     if (!m_frame->client())
         return;
     // No more events will be dispatched so detach the Document.
     if (m_frame->document())
         m_frame->document()->detach();
@@ skipping 446 matching lines @@
     // FIXME: We need a way to propagate insecure requests policy flags to
     // out-of-process frames. For now, we'll always use default behavior.
     if (!parentFrame->isLocalFrame())
         return nullptr;
 
     ASSERT(toLocalFrame(parentFrame)->document());
     return toLocalFrame(parentFrame)->document()->insecureNavigationsToUpgrade();
 }
 
 } // namespace blink