Document attribute charset and its aliases to return UTF-8

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
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     // FIXME: The typeExtension arguments should not be nullable.
     [CustomElementCallbacks, PerWorldBindings, RaisesException] Element createElement(DOMString localName, DOMString? typeExtension);
     [CustomElementCallbacks, RaisesException] Element createElementNS(DOMString? namespaceURI, DOMString qualifiedName, DOMString? typeExtension);
 
     // Page Visibility
     // http://www.w3.org/TR/page-visibility/#sec-document-interface
     readonly attribute boolean hidden;
     readonly attribute VisibilityState visibilityState;
 
     // Non-standard APIs
-    [MeasureAs=DocumentCharset, TreatReturnedNullStringAs=Undefined] readonly attribute DOMString charset;
-    [MeasureAs=DocumentDefaultCharset, TreatReturnedNullStringAs=Undefined] readonly attribute DOMString defaultCharset;
+    [MeasureAs=DocumentCharset] readonly attribute DOMString charset;
+    [MeasureAs=DocumentDefaultCharset] readonly attribute DOMString defaultCharset;
     [MeasureAs=DocumentCaretRangeFromPoint] Range caretRangeFromPoint([Default=Undefined] optional long x, [Default=Undefined] optional long y);
     [DeprecateAs=DocumentGetCSSCanvasContext] RenderingContext getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);
 
     // Deprecated prefixed page visibility API.
     // TODO(davidben): This is a property so attaching a deprecation warning results in false positives when outputting
     // document in the console. It's possible http://crbug.com/43394 will resolve this.
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=visibilityState] readonly attribute DOMString webkitVisibilityState;
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=hidden] readonly attribute boolean webkitHidden;
 
     // Event handler attributes
     attribute EventHandler onbeforecopy;
     attribute EventHandler onbeforecut;
     attribute EventHandler onbeforepaste;
     attribute EventHandler oncopy;
     attribute EventHandler oncut;
     attribute EventHandler onpaste;
     attribute EventHandler onsearch;
     [RuntimeEnabled=ExperimentalContentSecurityPolicyFeatures] attribute EventHandler onsecuritypolicyviolation;
     attribute EventHandler onselectionchange;
     attribute EventHandler onselectstart;
     attribute EventHandler onwheel;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;
 Document implements NonElementParentNode;