chromium/chrome/browser/resources/settings/controls/settings_dropdown_menu.html

<style
<if expr="not chromeos_ash">
  include="md-select"
</if>
<if expr="chromeos_ash">
  include="md-select cros-color-overrides"
</if>
>
  :host {
    align-items: center;
    display: inline-flex;
  }

  /* When settings-dropdown-menu is start-aligned, we probably want policy
   * indicator to be be displayed after the dropdown.
   * Setting --settings-dropdown-menu-policy-order to 1 will do the job.
   */
  cr-policy-pref-indicator {
    height: var(--iron-icon-width, 24px);
    margin: 0 var(--cr-controlled-by-spacing);
    order: var(--settings-dropdown-menu-policy-order, 0);
    width: var(--iron-icon-width, 24px);
  }

  /* Hide "Custom" value when unselectable. */
  option:disabled {
    display: none;
  }
</style>
<template is="dom-if" if="[[pref.controlledBy]]" restamp>
  <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
</template>
<select class="md-select" id="dropdownMenu" on-change="onChange_"
    aria-label$="[[label]]" part="select"
    disabled="[[shouldDisableMenu_(disabled, menuOptions.*, pref.*)]]">
  <template is="dom-repeat" items="[[menuOptions]]">
    <option value="[[item.value]]" hidden="[[item.hidden]]">
      [[item.name]]
    </option>
  </template>
  <option value="[[notFoundValue]]"
      disabled="[[!showNotFoundValue_(menuOptions, pref.value)]]">
    $i18n{custom}
  </option>
</select>