Add failure messages to gin and js mojom encoder

mojo/public/js/codec_unittests.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([
     "gin/test/expect",
     "mojo/public/js/codec",
     "mojo/public/interfaces/bindings/tests/rect.mojom",
     "mojo/public/interfaces/bindings/tests/sample_service.mojom",
     "mojo/public/interfaces/bindings/tests/test_structs.mojom",
   ], function(expect, codec, rect, sample, structs) {
   testBar();
   testFoo();
   testNamedRegion();
   testTypes();
   testAlign();
   testUtf8();
+  testArrayPointerValidation();
   this.result = "PASS";
 
   function testBar() {
     var bar = new sample.Bar();
     bar.alpha = 1;
     bar.beta = 2;
     bar.gamma = 3;
     bar.type = 0x08070605;
     bar.extraProperty = "banana";
 
@@ skipping 220 matching lines @@
     var actualMemory = new Uint8Array(message.buffer.arrayBuffer);
     expect(actualMemory.length).toEqual(expectedMemory.length);
     expect(actualMemory).toEqual(expectedMemory);
 
     var reader = new codec.MessageReader(message);
     expect(reader.payloadSize).toBe(payloadSize);
     expect(reader.messageName).toBe(messageName);
     var str2 = reader.decoder.decodeStringPointer();
     expect(str2).toEqual(str);
   }
+
+  function testArrayPointerValidation() {
+    function DummyClass() {};
+    var builder = new codec.MessageBuilder(42, 24);
+    var encoder = builder.createEncoder(8);
+    expect(function() {
+      // Pass something that is not an array it should throw.
+      encoder.encodeArrayPointer(DummyClass, 75);
+    }).toThrow();
+
+    // An empty array or null should do nothing and be fine.
+    encoder.encodeArrayPointer(DummyClass, []);
+    encoder.encodeArrayPointer(DummyClass, null);
+  }
 });