Set default document urls to 'about:blank'.

third_party/WebKit/Source/core/dom/Document.h

 /*
  * 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, 2010, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
@@ skipping 577 matching lines @@
              Document* enteredDocument = nullptr,
              ExceptionState& = ASSERT_NO_EXCEPTION);
   void writeln(const String& text,
                Document* enteredDocument = nullptr,
                ExceptionState& = ASSERT_NO_EXCEPTION);
   void write(LocalDOMWindow*, const Vector<String>& text, ExceptionState&);
   void writeln(LocalDOMWindow*, const Vector<String>& text, ExceptionState&);
 
   bool wellFormed() const { return m_wellFormed; }
 
+  // Get the URL that was used to retrieve this document.

[tkent, 2017/03/22 08:36:27]

We are not sure if we should remove it or not. So past tense isn't reasonable.
Also, this comment doesn't explain difference from urlForBinding().
How about something like:

// Return the document URL, or an empty URL if it's unavailable.
// This is not an implementation of web-exposed Document.prototype.URL.

   const KURL& url() const { return m_url; }
   void setURL(const KURL&);
 
+  // Bind the url to document.url, if unavailable bind to about:blank.
+  KURL urlForBinding();
+
   // To understand how these concepts relate to one another, please see the
   // comments surrounding their declaration.
   const KURL& baseURL() const { return m_baseURL; }
   void setBaseURLOverride(const KURL&);
   const KURL& baseURLOverride() const { return m_baseURLOverride; }
   KURL validBaseElementURL() const;
   const AtomicString& baseTarget() const { return m_baseTarget; }
   void processBaseElement();
 
   // Creates URL based on passed relative url and this documents base URL.
@@ skipping 1096 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif  // Document_h