Make ResourceFetcher return Resources with LoadError instead of nullptrs.

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 e4caac40bb90dc83ab950f47f4609b065ff58444..abe32fc82d15ac6a3e8c76cf9cdd278fcaee9045 100644
--- a/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -1365,7 +1365,12 @@ void XMLHttpRequest::didFail(const ResourceError& error)
     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;
     }