Fix ChromeVox next tests to fail instead of timing out where applicable.

chrome/browser/resources/chromeos/chromevox/testing/callback_helper.js

+// Copyright 2015 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.
+
+/**
+ * Creates wrappers for callbacks and calls testDone() when all callbacks
+ * have been invoked.
+ * @param {testing.Test} fixture
+ */
+function CallbackHelper(fixture) {
+  /** @type {Object} fixture */
+  this.fixture_ = fixture;
+  /** @type {number} */
+  this.pendingCallbacks_ = 0;
+}
+
+CallbackHelper.prototype = {
+  /**
+   * @param {Function=} opt_callback
+   * @return {Function}
+   */
+  wrap: function(opt_callback) {
+    var callback = opt_callback || function() {};
+    var savedArgs = new SaveMockArguments();
+    var lastCall = null;
+    var completionAction = callFunctionWithSavedArgs(savedArgs, function() {
+      if (lastCall) {
+        throw new Error('Called more than once, first call here: ' + lastCall);
+      } else {
+        lastCall = new Error().stack;
+      }
+      callback.apply(this.fixture_, arguments);
+      if (--this.pendingCallbacks_ <= 0)
+        testDone();
+    }.bind(this));
+    // runAllActionsAsync catches exceptions and puts them in the test
+    // framework's list of errors and fails the test.
+    var runAll = runAllActionsAsync(WhenTestDone.ASSERT, completionAction);
+    ++this.pendingCallbacks_;
+    return function() {
+      savedArgs.arguments = Array.prototype.slice.call(arguments);
+      runAll.invoke();
+    }
+  }
+};