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..dbfc7f853e3cbcfe29bf5ecbacc74b5e30494878 100644
--- a/Source/core/xml/XMLHttpRequest.cpp
+++ b/Source/core/xml/XMLHttpRequest.cpp
@@ -590,6 +590,13 @@ 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
+        // Use count for XHR synchronous requests on main thread only.
+        if (!document()->processingBeforeUnload())

[kouhei (in TOK), 2014/10/07 23:21:18]

Ditto: !unloadStarted() ?

+            UseCounter::countDeprecation(executionContext(), UseCounter::XMLHttpRequestSynchronous);

[kouhei (in TOK), 2014/10/07 23:21:18]

I don't think we should move callsite of existing UseCounter, as then we would
get mixed results from Chrome before/after this change. Use a new one?

[Mayur Kankanwadi, 2014/10/08 10:14:25]

If we use UseCounter::countDeprecation here, then the subsequent
UseCounter::count asserts. So the option was to remove this call and let the
below UseCounter::count to continue. 
But then we want to show the deprecation message only for main thread sync xhr,
worker thread sync xhr's are fine. 
Should we add one more counter like XMLHttpRequestSynchronousInWorker? So that
we can account for sync xhr's in all scenarios, probably in another CL!!

     }
 
     m_method = uppercaseKnownHTTPMethod(method);
@@ -879,11 +886,8 @@ void XMLHttpRequest::createRequest(PassRefPtr<FormData> httpBody, ExceptionState
         // FIXME: Maybe create() can return null for other reasons too?
         ASSERT(!m_loader);
         m_loader = ThreadableLoader::create(executionContext, this, request, options, resourceLoaderOptions);
-    } else {
-        // Use count for XHR synchronous requests.
-        UseCounter::count(&executionContext, UseCounter::XMLHttpRequestSynchronous);
+    } else

[kouhei (in TOK), 2014/10/07 23:21:18]

Ditto

         ThreadableLoader::loadResourceSynchronously(executionContext, request, *this, options, resourceLoaderOptions);
-    }
 
     if (!m_exceptionCode && m_error)
         m_exceptionCode = NetworkError;