chromium/chrome/browser/resources/side_panel/reading_list/icons.html

<cr-iconset name="read-later" size="24">
  <svg>
    <defs>
      <!-- Material icons -->
      <g id="check-circle-outline">
        <path d="M16.59 7.58L10 14.17l-3.59-3.58L5 12l5 5 8-8zM12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm0 18c-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8-3.58 8-8 8z">
        </path>
      </g>
    </defs>
</cr-iconset>