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 69986e041d2e82d180eab8f692b98c05f61ad237..ae0f34197145e87d80c08b2e41528329b4143652 100644
--- a/Source/core/xml/XMLHttpRequest.cpp
+++ b/Source/core/xml/XMLHttpRequest.cpp
@@ -590,6 +590,15 @@ 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
+        if (!executionContext()->inBeforeUnloadEvent() && !document()->didWarnAboutSyncXHROnce()) {

[Mike West, 2014/10/01 12:00:44]

You can avoid the "didWarnAbout..." flag by using UseCounter::countDeprecation.
In fact, there's already a counter defined for this:
'XMLHttpRequestSynchronous', and it's triggered in
XMLHttpRequest::createRequest:884.

I'd suggest modifying that callsite to use '::countDeprecation()' rather than
'::count()', and adding your deprecation message to the list in
'UseCounter::deprecationMessage'.

[Mayur Kankanwadi, 2014/10/06 12:48:16]

Done.

+            document()->setWarnedAboutSyncXHROnce();
+            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."));
+        }
     }
 
     m_method = uppercaseKnownHTTPMethod(method);