Make ResourceFetcher return Resources with LoadError instead of nullptrs.

third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php

Index: third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php
diff --git a/third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php b/third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php
index 2f134fc303d35e955dcfc4a0bea2642233bde055..5b3d138a2fbcd4736edf682b66699259d39703d3 100644
--- a/third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php
+++ b/third_party/WebKit/LayoutTests/http/tests/security/contentSecurityPolicy/nonces/import-enforce-blocked.php
@@ -5,23 +5,33 @@
 <script nonce="abc" src="/resources/testharness.js"></script>
 <script nonce="abc" src="/resources/testharnessreport.js"></script>
 <script nonce="abc">
+    // HTML imports are render-blocking, therefore the test must not complete
+    // before the import finishes (or fails) loading, otherwise, the test output
+    // will be empty (or at least flaky).
+    //
+    // This is because testharness.js appends the results as <pre> elements to the
+    // DOM, and then dumps the document as text. However, in a non-rendering-ready
+    // document, all elements are effectively `display: none`, and as such would
+    // not appear in the dump.
     async_test(t => {
         window.onload = t.step_func(_ => {
             var link = document.createElement('link');
             link.setAttribute("rel", "import");
             link.setAttribute("nonce", "zyx");
             link.setAttribute("href", "/security/resources/blank.html");
-
-            var watcher = new EventWatcher(t, document, ['securitypolicyviolation']);
-            watcher
-                .wait_for('securitypolicyviolation')
-                .then(t.step_func_done(e => {
-                    assert_false(!!link.import, "The import was not loaded.");
-                    assert_equals(e.blockedURI, "http://127.0.0.1:8000/security/resources/blank.html");
-                    assert_equals(e.lineNumber, 21);
-                 }));
-
+            link.onload = t.unreached_func();
+            link.onerror = t.step_func_done();
             document.body.appendChild(link);
         });
-    }, "Incorrectly nonced imports do not load, and trigger reports.");
+    }, "Incorrectly nonced imports do not load.");
+
+    async_test(t => {
+        var watcher = new EventWatcher(t, document, ['securitypolicyviolation']);
+        watcher
+            .wait_for('securitypolicyviolation')
+            .then(t.step_func_done(e => {
+                assert_equals(e.blockedURI, "http://127.0.0.1:8000/security/resources/blank.html");
+                assert_equals(e.lineNumber, 21);
+             }));
+    }, "Incorrectly nonced imports trigger reports.");
 </script>