Refactor frame navigation/detach state cleanup to be more sane.

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 2143 matching lines @@
     // notifyContextDestroyed() for a document that doesn't get detached.
     // If such a document has any observer, the observer won't get
     // a contextDestroyed() notification. This can happen for a document
     // created by DOMImplementation::createDocument().
     DocumentLifecycleNotifier::notifyContextDestroyed();
     ExecutionContext::notifyContextDestroyed();
 }
 
 void Document::prepareForDestruction()
 {
-    m_markers->prepareForDestruction();
-    disconnectDescendantFrames();
+    ASSERT(!m_frame || m_frame->tree().childCount() == 0);

[dcheng, 2015/04/03 14:35:45]

If we were active at some point, all children must be detached before calling
us. This eliminates the insanity of "sometimes detachChildren() detaches the sub
frames, but sometimes prepareForDisconnection() does it just to catch cases that
were missed".

 
-    // The process of disconnecting descendant frames could have already detached us.
     if (!isActive())

[dcheng, 2015/04/03 14:35:45]

We still need to check this, due to the magic of nested synchronous navigations
to javascript: URLs in pagehide handlers.

[Nate Chapin, 2015/04/03 19:27:31]

D:

         return;
 
+    m_markers->prepareForDestruction();

[dcheng, 2015/04/03 14:35:45]

Moving this here seemed to be a no-op, and makes the code a bit nicer in that
nothing happens if the Document is already inactive.

+
     if (LocalDOMWindow* window = this->domWindow())
         window->willDetachDocumentFromFrame();
     detach();
 }
 
 void Document::removeAllEventListeners()
 {
     ContainerNode::removeAllEventListeners();
 
     if (LocalDOMWindow* domWindow = this->domWindow())
@@ skipping 3516 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif