Remove the Element.prefix setter

Source/core/frame/UseCounter.h

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 212 matching lines @@
         HTMLElementOuterText = 214,
         ReplaceDocumentViaJavaScriptURL = 215,
         ElementSetAttributeNodeNS = 216, // Removed from DOM4.
         ElementPrefixedMatchesSelector = 217,
         CSSStyleSheetRules = 219,
         CSSStyleSheetAddRule = 220,
         CSSStyleSheetRemoveRule = 221,
         // The above items are available in M33 branch.
 
         InitMessageEvent = 222,
-        ElementSetPrefix = 224, // Element.prefix is readonly in DOM4.
         CSSStyleDeclarationGetPropertyCSSValue = 225,
         PrefixedMediaCancelKeyRequest = 229,
         DOMImplementationHasFeature = 230,
         DOMImplementationHasFeatureReturnFalse = 231,
         CanPlayTypeKeySystem = 232,
         PrefixedDevicePixelRatioMediaFeature = 233,
         PrefixedMaxDevicePixelRatioMediaFeature = 234,
         PrefixedMinDevicePixelRatioMediaFeature = 235,
         PrefixedTransform3dMediaFeature = 237,
         PrefixedStorageQuota = 240,
@@ skipping 389 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h