Change the battery-status layout tests to use JS mocks.

third_party/WebKit/LayoutTests/resources/mojo-helpers.js

 /*
  * mojo-helpers contains extensions to testharness.js useful for consuming
  * and mocking Mojo services directly within test code.
  */
 'use strict';
 
 // Fix up the global window.define, since all baked-in Mojo modules expect to
 // find it there. This define() also returns a promise to the module.
 let define = (function(){
   let moduleCache = new Map();
@@ skipping 16 matching lines @@
   }
 })();
 
 define('Mojo Helpers', [
     'mojo/public/js/core',
     'mojo/public/js/router',
     'mojo/public/js/support',
     'content/public/renderer/frame_service_registry',
     'content/public/renderer/service_registry',
 ], (core, router, support, frameServiceRegistry, serviceRegistry) => {
-  add_completion_callback(() => {
+  let tearDown = () => {
     frameServiceRegistry.clearServiceOverridesForTesting();
     serviceRegistry.clearServiceOverridesForTesting();
-  });
+  };
+  addEventListener('unload', tearDown);
+  if (window.add_completion_callback)
+    add_completion_callback(tearDown);
 
   return {
     core,
     router,
     support,
     frameServiceRegistry,
     serviceRegistry,
   };
 });
 
@@ skipping 29 matching lines @@
       let buffer, handles;
       ({ result, buffer, handles } = mojo.core.readMessage(pipe, 0));
       if (result !== mojo.core.RESULT_OK) {
         reject(result);
         return;
       }
       resolve({ buffer, handles });
     });
   });
 };