chromium/components/browser_ui/widget/android/java/res/drawable/tile_view_highlight_mask.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
     Copyright 2015 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"
    android:shape="oval" >
    <solid android:color="#000000" />
</shape>