First set of WebUSB layout tests with Mojo service 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.
-function define(name, deps, factory) {
-  return new Promise(resolve => {
-    mojo.define(name, deps, (...modules) => {
-      let result = factory(...modules);
-      resolve(result);
-      return result;
-    });
-  });
-}
+let define = (function(){
+  let moduleCache = new Map();
+
+  return function(name, deps, factory) {
+    let promise = moduleCache.get(name);
+    if (promise === undefined) {
+      // This promise must be cached as mojo.define will only call the factory
+      // function the first time the module is defined.
+      promise = new Promise(resolve => {
+        mojo.define(name, deps, (...modules) => {
+          let result = factory(...modules);
+          resolve(result);
+          return result;
+        });
+      });
+      moduleCache.set(name, promise);
+    }
+    return promise;
+  }
+})();
 
 // Returns a promise to an object that exposes common Mojo module interfaces.
 // Additional modules to load can be specified in the |modules| parameter. The
 // result will contain them, in the same order, in the |modules| field.
 function loadMojoModules(name, modules = []) {
   return define('Mojo layout test module: ' + name, [
       'mojo/public/js/core',
       'mojo/public/js/router',
       'mojo/public/js/support',
       'content/public/renderer/service_provider',
@@ skipping 39 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 });
     });
   });
 };