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..054faaed26589f917ab5bf3442e8ad7b145000c0 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

[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 + "\"");