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

Source/modules/fetch/FetchHeaderList.cpp

Index: Source/modules/fetch/FetchHeaderList.cpp
diff --git a/Source/modules/fetch/FetchHeaderList.cpp b/Source/modules/fetch/FetchHeaderList.cpp
index a03e9f3e3d39d1b679f12d5ba463588338253050..b5784545da3e84a30cb5df9e6e1a4fa10f12d4e9 100644
--- a/Source/modules/fetch/FetchHeaderList.cpp
+++ b/Source/modules/fetch/FetchHeaderList.cpp
@@ -6,6 +6,7 @@
 #include "modules/fetch/FetchHeaderList.h"
 
 #include "core/fetch/FetchUtils.h"
+#include "core/frame/UseCounter.h"
 #include "platform/network/HTTPParsers.h"
 #include "wtf/PassOwnPtr.h"
 
@@ -153,6 +154,11 @@ bool FetchHeaderList::isValidHeaderValue(const String& value)
 {
     // "A value is a byte sequence that matches the field-value token production
     // and contains no 0x0A or 0x0D bytes."
+
+    // Show deprecation warnings and count occurrences of such deprecated header values
+    if (!isValidHTTPFieldContentRFC7230(value) && isValidHTTPHeaderValue(value))
+        UseCounter::deprecationMessage(UseCounter::HeaderValueNotMatchingRFC7230);
+
     return isValidHTTPHeaderValue(value);
 }