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

Source/core/xmlhttprequest/XMLHttpRequest.cpp

Index: Source/core/xmlhttprequest/XMLHttpRequest.cpp
diff --git a/Source/core/xmlhttprequest/XMLHttpRequest.cpp b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
index 4ec6c06ee3273aa78025da1f4d662b3148431ebc..582385923a8717d77b2fde59f94499c20f3c1d7b 100644
--- a/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -1183,6 +1183,10 @@ void XMLHttpRequest::setRequestHeader(const AtomicString& name, const AtomicStri
         return;
     }
 
+    // Show deprecation warnings and count occurrences of such deprecated header values.
+    if (!value.isEmpty() && !isValidHTTPFieldContentRFC7230(value))
+        UseCounter::countDeprecation(executionContext(), UseCounter::HeaderValueNotMatchingRFC7230);
+
     // No script (privileged or not) can set unsafe headers.
     if (FetchUtils::isForbiddenHeaderName(name)) {
         logConsoleError(executionContext(), "Refused to set unsafe header \"" + name + "\"");