Show a warning when using sync xhr.

Source/core/xml/XMLHttpRequest.cpp

Index: Source/core/xml/XMLHttpRequest.cpp
diff --git a/Source/core/xml/XMLHttpRequest.cpp b/Source/core/xml/XMLHttpRequest.cpp
index 7fe4b47433d6a577db730aa2ccf75c78053f12e0..a901d2a4137b57fade5987ac6edd6063f52774d9 100644
--- a/Source/core/xml/XMLHttpRequest.cpp
+++ b/Source/core/xml/XMLHttpRequest.cpp
@@ -592,6 +592,12 @@ void XMLHttpRequest::open(const AtomicString& method, const KURL& url, bool asyn
             exceptionState.throwDOMException(InvalidAccessError, "Synchronous requests must not set a timeout.");
             return;
         }
+
+        // Here we just warn that firing sync XHR's may affect responsiveness.
+        // Eventually sync xhr will be deprecated and an "InvalidAccessError" exception thrown.
+        // Refer : https://xhr.spec.whatwg.org/#sync-warning
+        executionContext()->addConsoleMessage(ConsoleMessage::create(JSMessageSource, WarningMessageLevel
+            , "Avoid synchronous requests as it hampers user experience due to unresponsiveness of the web page.Refer https://xhr.spec.whatwg.org/#sync-warning."));

[tyoshino (SeeGerritForStatus), 2014/09/19 11:03:10]

Insert a space between "page." and "Refer"

Please break the line after ",".

[Mayur Kankanwadi, 2014/09/19 11:24:17]

Done.

     }
 
     m_method = uppercaseKnownHTTPMethod(method);