chromium/chrome/browser/tab_ui/android/java/res/drawable/tab_strip_favicon_circle.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2020 The Chromium Authors
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->

<layer-list
xmlns:android="http://schemas.android.com/apk/res/android" >
    <item
        android:width="20dp"
        android:height="20dp"
        android:gravity="center">
        <shape
            android:shape="oval">
            <solid android:color="@color/default_bg_color_light"/>
        </shape>
    </item>
</layer-list>