Remove Node.prefix so it's no longer visible to script.

Source/core/dom/Element.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *           (C) 2007 Eric Seidel (eric@webkit.org)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 55 matching lines @@
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/editing/FrameSelection.h"
 #include "core/editing/TextIterator.h"
 #include "core/editing/htmlediting.h"
 #include "core/editing/markup.h"
 #include "core/events/EventDispatcher.h"
 #include "core/events/FocusEvent.h"
 #include "core/frame/ContentSecurityPolicy.h"
 #include "core/frame/Frame.h"
 #include "core/frame/FrameView.h"
+#include "core/frame/UseCounter.h"
 #include "core/html/ClassList.h"
 #include "core/html/HTMLCollection.h"
 #include "core/html/HTMLDocument.h"
 #include "core/html/HTMLElement.h"
 #include "core/html/HTMLFormControlsCollection.h"
 #include "core/html/HTMLFrameOwnerElement.h"
 #include "core/html/HTMLLabelElement.h"
 #include "core/html/HTMLOptionsCollection.h"
 #include "core/html/HTMLTableRowsCollection.h"
 #include "core/html/HTMLTemplateElement.h"
@@ skipping 1139 matching lines @@
     return m_tagName.toString();
 }
 
 String Element::nodeNamePreservingCase() const
 {
     return m_tagName.toString();
 }
 
 void Element::setPrefix(const AtomicString& prefix, ExceptionState& exceptionState)
 {
+    UseCounter::count(document(), UseCounter::ElementSetPrefix);
+
     checkSetPrefix(prefix, exceptionState);
     if (exceptionState.hadException())
         return;
 
     m_tagName.setPrefix(prefix.isEmpty() ? AtomicString() : prefix);
 }
 
 KURL Element::baseURI() const
 {
     const AtomicString& baseAttribute = getAttribute(baseAttr);
@@ skipping 2373 matching lines @@
     // Before doing so, we need to resolve issues in HTMLSelectElement::recalcListItems
     // and RenderMenuList::setText. See also https://bugs.webkit.org/show_bug.cgi?id=88405
     if (hasTagName(optionTag) || hasTagName(optgroupTag))
         return false;
     if (FullscreenElementStack::isActiveFullScreenElement(this))
         return false;
     return true;
 }
 
 } // namespace WebCore