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

Source/core/xmlhttprequest/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 1165 matching lines @@
     if (!isValidHTTPToken(name)) {
         exceptionState.throwDOMException(SyntaxError, "'" + name + "' is not a valid HTTP header field name.");
         return;
     }
 
     if (!isValidHTTPHeaderValue(value)) {
         exceptionState.throwDOMException(SyntaxError, "'" + value + "' is not a valid HTTP header field value.");
         return;
     }
 
+    // Show deprecation warnings and count occurrences of such deprecated header values

[hiroshige, 2015/09/01 11:21:18]

nit: append '.' at the end of the statement.

[shiva.jm, 2015/09/01 14:29:11]

Done.

+    if (!isValidHTTPFieldContentRFC7230(value))

[hiroshige, 2015/09/01 11:21:18]

After I saw the diff of xmlhttprequest-setrequestheader-no-value-expected.txt, I
found the empty string is expected to be a valid header value, and I filed
https://github.com/whatwg/fetch/issues/115.
So I expect this will be:
if (!value.isEmpty() && !isValidHTTPFieldContentRFC7230(value))
If this change is done, I expect the warning diff in
LayoutTests/http/tests/xmlhttprequest/xmlhttprequest-setrequestheader-no-value-expected.txt
can be removed.

[shiva.jm, 2015/09/01 14:29:11]

Done.

+        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 + "\"");
         return;
     }
 
     setRequestHeaderInternal(name, value);
 }
 
 void XMLHttpRequest::setRequestHeaderInternal(const AtomicString& name, const AtomicString& value)
@@ skipping 497 matching lines @@
     visitor->trace(m_responseDocumentParser);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     visitor->trace(m_blobLoader);
     XMLHttpRequestEventTarget::trace(visitor);
     DocumentParserClient::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink