chrome/android/java/res/drawable/ntp_toolbar_button_background.xml

<?xml version="1.0" encoding="utf-8"?>

<!-- Copyright 2014 The Chromium Authors. All rights reserved.

     Use of this source code is governed by a BSD-style license that can be
     found in the LICENSE file.
-->

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true"
          android:drawable="@drawable/ntp_toolbar_button_background_pressed_img" /> <!-- pressed -->
</selector>