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

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2019 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:left="6dp"
        android:top="6dp"
        android:right="6dp"
        android:bottom="6dp">
        <shape
            android:shape="oval">
            <solid android:color="@color/favicon_background_color" />
        </shape>
    </item>
</layer-list>