Add a use counter for async XHR.

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 3fd2c3fc7ef2eda0dd9133cbce9fb6e680cbcd7e..da958dde17562d0f17df1f7dcf36ffb66690d442 100644
--- a/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -1000,6 +1000,7 @@ void XMLHttpRequest::createRequest(PassRefPtr<FormData> httpBody, ExceptionState
     m_error = false;
 
     if (m_async) {
+        UseCounter::count(&executionContext, UseCounter::XMLHttpRequestAsynchronous);

[tyoshino (SeeGerritForStatus), 2015/01/27 05:07:42]

This counts sendForInspectorXHRReplay(), too. It's for DevTools. But maybe it's
ok as on that page there the XHR has already happened and was counted, so
sendForInspectorXHRReplay() would be no-op regarding UseCounter::count().

Also we already have one for sync XHR at L1015. Let's just align with it.

         if (m_upload)
             request.setReportUploadProgress(true);