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..0380148b6d28e71c216b4f1dc2b62e2f0e12fcae 100644
--- a/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -1178,6 +1178,10 @@ void XMLHttpRequest::setRequestHeader(const AtomicString& name, const AtomicStri
         return;
     }
 
+    // Show deprecation warnings and count occurrences of such deprecated header values
+    if (isValidHTTPHeaderValueForFetch(value))

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

How about replacing this with "if (!isValidHTTPHeaderValueForFetch(value))" and
placing this after the "if (!isValidHTTPHeaderValue(value)) {...}" block?

i.e. we want to warn header values that pass
isValidHTTPHeaderValue() but don't pass
isValidHTTPHeaderValueForFetch().

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

Done.

+        UseCounter::countDeprecation(executionContext(), UseCounter::FetchHeaderValue);
+
     if (!isValidHTTPHeaderValue(value)) {
         exceptionState.throwDOMException(SyntaxError, "'" + value + "' is not a valid HTTP header field value.");
         return;