Add CHECKs to make sure ThreadableLoader has not been created multiple times

third_party/WebKit/Source/modules/eventsource/EventSource.cpp

 /*
  * Copyright (C) 2009, 2012 Ericsson AB. All rights reserved.
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011, Code Aurora Forum. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 155 matching lines @@
       (origin->canRequestNoSuborigin(m_currentURL) || m_withCredentials)
           ? AllowStoredCredentials
           : DoNotAllowStoredCredentials;
   resourceLoaderOptions.credentialsRequested =
       m_withCredentials ? ClientRequestedCredentials
                         : ClientDidNotRequestCredentials;
   resourceLoaderOptions.dataBufferingPolicy = DoNotBufferData;
   resourceLoaderOptions.securityOrigin = origin;
 
   InspectorInstrumentation::willSendEventSourceRequest(&executionContext, this);
+  // TODO(yhirano): Remove this CHECK once https://crbug.com/667254 is fixed.
+  CHECK(!m_loader);
   // InspectorInstrumentation::documentThreadableLoaderStartedLoadingForClient
   // will be called synchronously.
   m_loader = ThreadableLoader::create(executionContext, this, options,
                                       resourceLoaderOptions);
   m_loader->start(request);
 }
 
 void EventSource::networkRequestEnded() {
   InspectorInstrumentation::didFinishEventSourceRequest(getExecutionContext(),
                                                         this);
@@ skipping 190 matching lines @@
 
 DEFINE_TRACE(EventSource) {
   visitor->trace(m_parser);
   visitor->trace(m_loader);
   EventTargetWithInlineData::trace(visitor);
   ContextLifecycleObserver::trace(visitor);
   EventSourceParser::Client::trace(visitor);
 }
 
 }  // namespace blink