The documentation for paper-input-behavior.html says that `list` should be of type String and should match the id of an existing <datalist> element. So you would have to create HTML for the datalist element, and then in your <input> element set `list="<id>"` and `autocomplete="on"`, where <id> is the id of your datalist.
<datalist id="countries-list">
<template is="dom-repeat" list="[[countries]]">
<option value="[[item]]">
<paper-input list="countries-list" autocomplete="on"></paper-input>