Add a deprecation warning for document.clear()

Source/core/page/UseCounter.cpp

 /*
  * 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 127 matching lines @@
     // Performance
     case PrefixedPerformanceTimeline:
         return "'window.performance.webkitGet*' methods have been deprecated. Please use the unprefixed 'performance.get*' methods instead.";
     case PrefixedUserTiming:
         return "'window.performance.webkit*' methods have been deprecated. Please use the unprefixed 'window.performance.*' methods instead.";
 
     // Web Audio
     case WebAudioLooping:
         return "AudioBufferSourceNode 'looping' attribute is deprecated.  Use 'loop' instead.";
 
+    case DocumentClear:
+        return "document.clear() is deprecated. This method doesn't do anything.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();
     }
 }
 
 } // namespace WebCore