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 4be6424aa5c6e6fef62172fee654c8d23926633e..1ecf0f9f5763e14a241caeaae71145baaaaaf29d 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -935,6 +935,12 @@ String UseCounter::deprecationMessage(Feature feature)
     case SVGSMILAnimationInImageRegardlessOfCache:
         return "SVG's SMIL animations (<animate>, <set>, etc.) are deprecated and will be removed. Please use CSS animations or Web animations instead.";
 
+    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();