Deprecate the Document.charset setter

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index b8ca2d69568a73cf0f982764bfc3c9cc44db2326..5fbb2a824762479f2a3e6b66c023bc3986eb78d5 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -802,6 +802,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case GetMatchedCSSRules:
         return "'getMatchedCSSRules()' is deprecated. For more help, check https://code.google.com/p/chromium/issues/detail?id=437569#c2";
 
+    case DocumentSetCharset:
+        return "Setting 'Document.charset' is deprecated. Please use '<meta charset=\"UTF-8\">' instead.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();