Add about:flags support for doc.write script blocking.

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)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 915 matching lines @@
     void serviceScriptedAnimations(double monotonicAnimationStartTime);
 
     int requestIdleCallback(IdleRequestCallback*, const IdleRequestOptions&);
     void cancelIdleCallback(int id);
 
     EventTarget* errorEventTarget() final;
     void logExceptionToConsole(const String& errorMessage, int scriptId, const String& sourceURL, int lineNumber, int columnNumber, PassRefPtr<ScriptCallStack>) final;
 
     void initDNSPrefetch();
 
-    bool isInDocumentWrite() { return m_writeRecursionDepth > 0; }
+    bool isInDocumentWrite() const { return m_writeRecursionDepth > 0; }
 
     TextAutosizer* textAutosizer();
 
     RawPtr<Element> createElement(const AtomicString& localName, const AtomicString& typeExtension, ExceptionState&);
     RawPtr<Element> createElementNS(const AtomicString& namespaceURI, const AtomicString& qualifiedName, const AtomicString& typeExtension, ExceptionState&);
     ScriptValue registerElement(ScriptState*, const AtomicString& name, const ElementRegistrationOptions&, ExceptionState&, CustomElement::NameSet validNames = CustomElement::StandardNames);
     CustomElementRegistrationContext* registrationContext() { return m_registrationContext.get(); }
     CustomElementMicrotaskRunQueue* customElementMicrotaskRunQueue();
 
     void setImportsController(HTMLImportsController*);
@@ skipping 490 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