Mark DOMStringList.contains() as deprecated, add console warning.

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 794e5246280616c7eac16ece4dcd6f35f3e28642..2b7f014227ea74cca7480d6d26d5a3d2cb520d57 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -928,6 +928,12 @@ String UseCounter::deprecationMessage(Feature feature)
     case PrefixedMouseEventMovementY:
         return replacedBy("webkitMovementY", "movementY");
 
+    case DOMStringListContains:
+        return "'DOMStringList.contains()' is deprecated, and will be removed.";
+
+    case DOMStringListItem:
+        return "'DOMStringList.item(n)' is deprecated, and will be removed. Please use indexing instead, e.g.: list[n]";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();