Make some fields non-nullable in serial.mojom and data_stream.mojom.

extensions/renderer/resources/serial_service.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 define('serial_service', [
     'content/public/renderer/service_provider',
     'data_receiver',
     'data_sender',
     'device/serial/serial.mojom',
     'mojo/public/js/bindings/core',
@@ skipping 12 matching lines @@
    * converts parameters and result between the Apps serial API types and the
    * Mojo types.
    */
 
   var service = new serialMojom.SerialServiceProxy(new routerModule.Router(
       serviceProvider.connectToService(serialMojom.SerialServiceProxy.NAME_)));
 
   function getDevices() {
     return service.getDevices().then(function(response) {
       return $Array.map(response.devices, function(device) {
-        var result = {path: device.path || ''};
+        var result = {path: device.path};
         if (device.has_vendor_id)
           result.vendorId = device.vendor_id;
         if (device.has_product_id)
           result.productId = device.product_id;
         if (device.display_name)
           result.displayName = device.display_name;
         return result;
       });
     });
   }
@@ skipping 373 matching lines @@
 
   return {
     getDevices: getDevices,
     createConnection: Connection.create,
     getConnection: getConnection,
     getConnections: getConnections,
     // For testing.
     Connection: Connection,
   };
 });