Show deprecation warnings for header values in XHR according to RFC 7230

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 31cce9336d8713e1a078aa55c1c14b4ddf403713..a49748ca6abb55a8f6e1e1e70ce6a8a4ffa826d1 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -972,6 +972,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case FetchAPIRequestContext:
         return "Request.context is deprecated and will be removed in M46 (see: https://www.chromestatus.com/feature/5534702526005248).";
 
+    case HeaderValueNotMatchingRFC7230:
+        return "Header values not matching to RFC 7230, will be deprecated (see: https://www.chromestatus.com/feature/6457425448140800).";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();