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chromium/ash/webui/sample_system_web_app_ui/resources/trusted/inter_frame_communication.html 

<!-- Copyright 2020 The Chromium Authors
     Use of this source code is governed by a BSD-style license that can be
     found in the LICENSE file. -->
<!DOCTYPE html>
<meta charset="utf-8">
<title>Sample System Web App</title>
<iframe width="640" height="180" src="chrome-untrusted://sample-system-web-app/untrusted.html"></iframe>
<p>Task from child frame: <code id="child-task"></code></p>
<script type="module" src="inter_frame_communication.js"></script>

<!-- Below mojo script required to run browser tests -->
<script src="chrome://resources/mojo/mojo/public/js/mojo_bindings_lite.js"></script>