chromium/chrome/android/features/tab_ui/java/res/drawable/tab_grid_dialog_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.
-->

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <solid android:color="@color/tab_grid_dialog_background_color"/>
    <corners android:radius="@dimen/tab_grid_dialog_bg_radius" />
</shape>