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

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 646a36b53a65645d7d145b0ce1565c7bb7008fb3..e6cab2248606b0a0d930868d1cffa4b04fb504ba 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -2161,13 +2161,13 @@ void Document::detach(const AttachContext& context)
 
 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();