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 6ace984dcaf16bd391489ca41ad5f3ce20705cda..cc74178ed1275b624f6ec90be1c78b0349a4af30 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -971,6 +971,9 @@ 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 FetchHeaderValue:
+        return "isValidHTTPHeaderValue() will be deprecated and will be removed in future.";

[hiroshige, 2015/08/27 11:38:07]

How about "Header values not matching RFC 7230 will be deprecated (see:
https://www.chromestatus.com/feature/6457425448140800)"?
This message is for XHR/Fetch API users so should be a warning about header
values.
Also I created the chromestatus entry.

[shiva.jm, 2015/09/01 08:26:51]

Done.

+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();