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 a49748ca6abb55a8f6e1e1e70ce6a8a4ffa826d1..ba695de43add4746e8f5a807c4bbfea90a496556 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -975,6 +975,12 @@ String UseCounter::deprecationMessage(Feature feature)
     case HeaderValueNotMatchingRFC7230:
         return "Header values not matching to RFC 7230, will be deprecated (see: https://www.chromestatus.com/feature/6457425448140800).";
 
+    case DOMStringListContains:
+        return "'DOMStringList.contains()' is deprecated, and will be removed.";
+
+    case V8DOMStringList_Item_Method:
+        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();