Serve mojo WebUI resources from the same origin as the WebUI itself.

content/test/data/web_ui_mojo_shell_test.js

Index: content/test/data/web_ui_mojo_shell_test.js
diff --git a/content/test/data/web_ui_mojo_shell_test.js b/content/test/data/web_ui_mojo_shell_test.js
index 1ed359933313cbaf90c48d6188b4ca0f79c031b7..d1b35dc1ad11ab1b6dcbe6fdc3175d11c8421d47 100644
--- a/content/test/data/web_ui_mojo_shell_test.js
+++ b/content/test/data/web_ui_mojo_shell_test.js
@@ -35,9 +35,8 @@ define('main', [
         function (services) {
           var test = connectToService(services, testMojom.TestMojoService);
           test.getRequestorURL().then(function(response) {
-            // The requestor URL seen by the app should be localhost because the
-            // connection comes from this script hosted on the test server.
-            domAutomationController.send(response.url == 'http://127.0.0.1/');
+            domAutomationController.send(
+                response.url == 'chrome://mojo-web-ui/');
           });
         },
         function (exposedServices) {});