Add a deprecation warning for document.clear()

Source/core/page/UseCounter.cpp

Index: Source/core/page/UseCounter.cpp
diff --git a/Source/core/page/UseCounter.cpp b/Source/core/page/UseCounter.cpp
index dbb3ab6df53579dfff2bcb7ff25a2dedfaabba81..c6100facd047516cf858f5b544b81c7e7e618263 100644
--- a/Source/core/page/UseCounter.cpp
+++ b/Source/core/page/UseCounter.cpp
@@ -145,6 +145,9 @@ String UseCounter::deprecationMessage(Feature feature)
     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();