chromium/chrome/android/java/res/drawable/material_tooltip_background.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2016 The Chromium Authors
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->

<shape
    xmlns:android="http://schemas.android.com/apk/res/android">
    <corners android:radius="2dp"/>
    <padding
        android:left="16dp"
        android:right="16dp"/>
    <solid android:color="#e6616161"/>
</shape>