chromium/chrome/android/java/res/drawable/bookmark_title_bar_shadow.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="rectangle" >

    <gradient
        android:angle="270"
        android:endColor="#00000000"
        android:startColor="#30000000"
        android:type="linear" />

</shape>