XMLHttpRequest: check if 'loadstart' handler cancelled send().

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

Index: third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
diff --git a/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp b/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
index 05b63335d2f0e5735d28299f77abdd2d72aa2559..2777ed0d05d888388aea3e52fcc39856c2748243 100644
--- a/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -941,10 +941,18 @@ void XMLHttpRequest::createRequest(PassRefPtr<EncodedFormData> httpBody,
     InspectorInstrumentation::asyncTaskScheduled(
         &executionContext, "XMLHttpRequest.send", this, true);
     dispatchProgressEvent(EventTypeNames::loadstart, 0, 0);
+    // Event handler could have invalidated this send operation,
+    // (re)setting the send flag and/or initiating another send
+    // operation; leave quietly if so.
+    if (!m_sendFlag || m_loader)
+      return;
     if (httpBody && m_upload) {
       uploadEvents = m_upload->hasEventListeners();
       m_upload->dispatchEvent(
           ProgressEvent::create(EventTypeNames::loadstart, false, 0, 0));
+      // See above.
+      if (!m_sendFlag || m_loader)
+        return;
     }
   }