Disallow certain blocking DOM calls during microtask execution.

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 615 matching lines @@
 bool XMLHttpRequest::initSend(ExceptionState& exceptionState)
 {
     if (!getExecutionContext())
         return false;
 
     if (m_state != OPENED || m_loader) {
         exceptionState.throwDOMException(InvalidStateError, "The object's state must be OPENED.");
         return false;
     }
 
+    if (!m_async && exceptionState.isolate() && v8::MicrotasksScope::IsRunningMicrotasks(exceptionState.isolate())) {
+        Deprecation::countDeprecation(getExecutionContext(), UseCounter::During_Microtask_SyncXHR);
+        if (RuntimeEnabledFeatures::disableBlockingMethodsDuringMicrotasksEnabled()) {
+            exceptionState.throwDOMException(InvalidAccessError, "Cannot send() synchronous requests during microtask execution.");
+            return false;
+        }
+    }
+
+
     m_error = false;
     return true;
 }
 
 void XMLHttpRequest::send(const ArrayBufferOrArrayBufferViewOrBlobOrDocumentOrStringOrFormData& body, ExceptionState& exceptionState)
 {
     InspectorInstrumentation::willSendXMLHttpRequest(getExecutionContext(), url());
 
     if (body.isNull()) {
         send(String(), exceptionState);
@@ skipping 1059 matching lines @@
     visitor->trace(m_responseArrayBuffer);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     visitor->trace(m_blobLoader);
     XMLHttpRequestEventTarget::trace(visitor);
     DocumentParserClient::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink