Mojom: Add 'default' keyword for initializing structs to non-null value.

mojo/apps/js/bindings/sample_service_unittests.js

 // Copyright 2013 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([
     "console",
     "mojo/apps/js/test/hexdump",
     "gin/test/expect",
     "mojo/public/interfaces/bindings/tests/sample_service.mojom",
     "mojo/public/interfaces/bindings/tests/sample_import.mojom",
@@ skipping 105 matching lines @@
     expect(defaults.a10).toBe(1234);
     expect(defaults.a11).toBe(true);
     expect(defaults.a12).toBe(false);
     expect(defaults.a13).toBe(123.25);
     expect(defaults.a14).toBe(1234567890.123);
     expect(defaults.a15).toBe(1E10);
     expect(defaults.a16).toBe(-1.2E+20);
     expect(defaults.a17).toBe(1.23E-20);
     expect(defaults.a20).toBe(sample.Bar.Type.TYPE_BOTH);
     expect(defaults.a21).toBeNull();
-    expect(defaults.a22).toBeNull();
+    expect(defaults.a22).toBeTruthy();
+    expect(defaults.a22.shape).toBe(imported.Shape.SHAPE_RECTANGLE);
+    expect(defaults.a22.color).toBe(imported2.Color.COLOR_BLACK);
     // TODO(vtl): crbug.com/375845
     // expect(defaults.a21).toBeNull();
     // expect(defaults.a22).toBeNull();
   }
 
   function ServiceImpl() {
   }
 
   ServiceImpl.prototype = Object.create(sample.ServiceStub.prototype);
 
@@ skipping 21 matching lines @@
   var serviceProxy = new sample.ServiceProxy(receiver);
 
   checkDefaultValues();
 
   var foo = makeFoo();
   checkFoo(foo);
 
   var port = 10;
   serviceProxy.frobinate(foo, sample.ServiceProxy.BazOptions.BAZ_EXTRA, port);
 });