chrome/browser/resources/whats_new/whats_new_app.html

<style include="cr-hidden-style">
  #content {
    border: none;
    height: 100%;
    width: 100%;
  }
</style>
<template is="dom-if" if="[[url_]]">
  <iframe id="content" src="[[url_]]"></iframe>
</template>