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

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017 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. -->

<!-- ToolbarPhone uses this drawable to transition between shadows when
     the omnibox focus changes. The bottom toolbar should not have a shadow
     when the omnibox is unfocused so this TransitionDrawable uses an empty
     shape for the focused drawable item.
     TODO(twellington): Remove this after UX has a spec'ed out what the
     bottom toolbar omnibox un/focus animations should look like. -->
<transition xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape
            android:shape="rectangle"
            android:gravity="top|fill_horizontal" >
        </shape>
    </item>
    <item>
        <bitmap
            android:src="@drawable/toolbar_shadow_focused"
            android:gravity="top|fill_horizontal" />
    </item>
</transition>