Make Android back work correctly in Elderberry.

chrome/android/java/res/values-v17/styles.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. -->
 
 <resources xmlns:tools="http://schemas.android.com/tools">
     <!-- Q: Why put style resources under values-v17/ ?
          A: Problem:
             1. paddingStart causes a crash on Galaxy Tab&Note b/8351339.
             2. So we wrote a build script(generate_v14_compatible_resources.py) to convert
@@ skipping 21 matching lines @@
         <!-- Navigation Transitions, requires API level 21 -->
         <item name="android:windowAllowEnterTransitionOverlap" tools:targetApi="21">false</item>
         <item name="android:windowAllowReturnTransitionOverlap" tools:targetApi="21">true</item>
         <item name="android:windowContentTransitions" tools:targetApi="21">true</item>
         <item name="android:windowEnterTransition" tools:targetApi="21">@transition/fade</item>
         <item name="android:windowExitTransition" tools:targetApi="21">@null</item>
         <item name="android:windowSharedElementEnterTransition" tools:targetApi="21">@transition/move_image</item>
         <item name="android:windowSharedElementExitTransition" tools:targetApi="21">@transition/move_image</item>
     </style>
 
+    <style name="TabbedModeTheme" parent="MainTheme">
+        <item name="android:windowBackground">@drawable/window_background</item>
+    </style>
+
     <!-- Web app themes -->
     <style name="WebappTheme" parent="MainTheme">
         <item name="android:windowBackground">@null</item>
         <item name="android:windowDisablePreview">true</item>
     </style>
     <style name="WebappSplashScreenTextTheme">
         <item name="android:textSize">24sp</item>
         <item name="android:ellipsize">end</item>
         <item name="android:textColor">@color/default_text_color</item>
         <item name="android:gravity">center</item>
@@ skipping 477 matching lines @@
     <style name="ToolbarMenuButtonTablet" parent="ToolbarButton">
         <item name="android:layout_width">43dp</item>
         <item name="android:paddingEnd">3.5dp</item>
     </style>
     <style name="AppMenuItem">
         <item name="android:paddingStart">16dp</item>
         <item name="android:paddingEnd">16dp</item>
         <item name="android:background">?attr/listChoiceBackgroundIndicator</item>
     </style>
 </resources>