Deprecate the Element.prefix 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 aad7c4993b5ed19717fd899a7b6a8109d9364957..a25946e078acdad385773c3e16865a4940cd90f3 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -730,6 +730,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case HTMLHeadElementProfile:
         return "'HTMLHeadElement.profile' is deprecated. The reflected attribute has no effect.";
 
+    case ElementSetPrefix:
+        return "Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();