Make new WebUSB implementation more resilient to garbage collection.

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

 'use strict';
 
 function assertRejectsWithError(promise, name, message) {
   return promise.then(() => {
     assert_unreached('expected promise to reject with ' + name);
   }, error => {
     assert_equals(error.name, name);
     if (message !== undefined)
       assert_equals(error.message, message);
   });
 }
 
 // TODO(reillyg): Remove when jyasskin upstreams this to testharness.js:
 // https://crbug.com/509058.
 function callWithKeyDown(functionCalledOnKeyPress) {
   return new Promise(resolve => {
     function onKeyPress() {
       document.removeEventListener('keypress', onKeyPress, false);
       resolve(functionCalledOnKeyPress());
     }
     document.addEventListener('keypress', onKeyPress, false);
 
     eventSender.keyDown(' ', []);
   });
 }
 
+function runGarbageCollection() {
+  // Run gc() as a promise.
+  return new Promise((resolve, reject) => {
+    GCController.collect();
+    setTimeout(resolve, 0);
+  });
+}
+
 function usbMocks(mojo) {
   return define('USB Mocks', [
     'device/usb/public/interfaces/chooser_service.mojom',
     'device/usb/public/interfaces/device_manager.mojom',
     'device/usb/public/interfaces/device.mojom',
   ], (chooserService, deviceManager, device) => {
     function assertDeviceInfoEquals(device, info) {
       assert_equals(device.guid, info.guid);
       assert_equals(device.usbVersionMajor, info.usb_version_major);
       assert_equals(device.usbVersionMinor, info.usb_version_minor);
@@ skipping 421 matching lines @@
 }
 
 function usb_test(func, name, properties) {
   mojo_test(mojo => usbMocks(mojo).then(usb => {
     let result = Promise.resolve(func(usb));
     let cleanUp = () => usb.mockDeviceManager.reset();
     result.then(cleanUp, cleanUp);
     return result;
   }), name, properties);
 }