Deprecate unused feature DOMImplementationCreateCSSStyleSheet

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 24fc31ac30192a0c872c5839f5c70863fc517299..744eacfb44e34d33b7944a141abc1e4ca6f6d15d 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -700,6 +700,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case PrefixedMediaSourceOpen:
         return "'WebKitMediaSource' is deprecated. Please use 'MediaSource' instead.";
 
+    case DOMImplementationCreateCSSStyleSheet:
+        return "'DOMImplementation.createCSSStyleSheet()' is deprecated.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();