Fixing iframe jank in the local omnibox popup.

ui/webui/resources/js/assert.js

+// Copyright (c) 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.
+
+/**
+ * @fileoverview Assertion support.
+ */
+
+/**
+ * Simple common assertion API
+ * @param {*} condition The condition to test.  Note that this may be used to
+ *     test whether a value is defined or not, and we don't want to force a
+ *     cast to Boolean.
+ * @param {string=} opt_message A message to use in any error.
+ */
+function assert(condition, opt_message) {
+  'use strict';
+  if (!condition) {
+    var msg = 'Assertion failed';
+    if (opt_message)
+      msg = msg + ': ' + opt_message;
+    throw new Error(msg);
+  }
+}