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 a1033f1465c4a53a786cc764e7cef9b52ad0ab7b..0642eec317246f934f0534d5e3222e248fdfcdd0 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -867,6 +867,12 @@ String UseCounter::deprecationMessage(Feature feature)
     case ServiceWorkerClientsGetAll:
         return "'Clients.getAll()' is deprecated and will be removed soon. Please use 'Clients.matchAll()' instead.";
 
+    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();