Make ResourceFetcher return Resources with LoadError instead of nullptrs.

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 1450 matching lines @@
 
 void XMLHttpRequest::didFail(const ResourceError& error) {
   NETWORK_DVLOG(1) << this << " didFail()";
   ScopedEventDispatchProtect protect(&m_eventDispatchRecursionLevel);
 
   // If we are already in an error state, for instance we called abort(), bail
   // out early.
   if (m_error)
     return;
 
-  if (error.isCancellation()) {
+  // Internally, access check violations are considered `cancellations`, but
+  // at least the mixed-content and CSP specs require them to be surfaced as
+  // network errors to the page. See:
+  //   [1] https://www.w3.org/TR/mixed-content/#algorithms,
+  //   [2] https://www.w3.org/TR/CSP3/#fetch-integration.
+  if (error.isCancellation() && !error.isAccessCheck()) {
     handleDidCancel();
     return;
   }
 
   if (error.isTimeout()) {
     handleDidTimeout();
     return;
   }
 
   // Network failures are already reported to Web Inspector by ResourceLoader.
@@ skipping 369 matching lines @@
   visitor->traceWrappers(m_responseLegacyStream);
   visitor->traceWrappers(m_responseDocument);
   visitor->traceWrappers(m_responseArrayBuffer);
 }
 
 std::ostream& operator<<(std::ostream& ostream, const XMLHttpRequest* xhr) {
   return ostream << "XMLHttpRequest " << static_cast<const void*>(xhr);
 }
 
 }  // namespace blink