Make Android back work correctly in Elderberry.

chrome/android/java/AndroidManifest.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 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. -->
 
 <!--
 Note: This is a jinja2 template, processed at build time into the final manifest.
 
 Blocks denoted with { % block some_name % }foo{ % endblock % } can be overridden
 by a child template that "extends" this file.
@@ skipping 120 matching lines @@
             android:resource="@xml/app_restrictions"/>
 
         <!-- Note: All activities directly or indirectly derived from ChromeActivity
              must specify android:hardwareAccelerated="false".
 
              Since this activity (shown in the launcher) and the application
              (shown in Android's Settings/Apps list) share the same label, we
              do not specify one here to allow it to inherit from the app. -->
         <activity android:name="org.chromium.chrome.browser.document.ChromeLauncherActivity"
             android:theme="@android:style/Theme.Translucent.NoTitleBar"
-            android:relinquishTaskIdentity="true">
+            android:relinquishTaskIdentity="true"
+            android:taskAffinity=""

[Maria, 2016/06/17 22:35:51]

The only thing I don't understand is why this doesn't separate regular
CustomTabActivity into a new task stack. Is it because of
relinquishTaskIdentity?

[Ted C, 2016/06/20 15:49:16]

That was only added in Lollipop, so it can't be that.  I wish I knew :-/

+            android:excludeFromRecents="true">
         </activity>
         <activity-alias android:name="com.google.android.apps.chrome.Main"
             android:targetActivity="org.chromium.chrome.browser.document.ChromeLauncherActivity"
             android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.DEFAULT" />
                 <category android:name="android.intent.category.LAUNCHER" />
                 <category android:name="android.intent.category.BROWSABLE" />
                 <category android:name="android.intent.category.APP_BROWSER" />
@@ skipping 122 matching lines @@
             android:theme="@style/MainTheme"
             android:exported="false"
             android:taskAffinity=""
             android:windowSoftInputMode="adjustResize"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize"
             android:hardwareAccelerated="false">
         </activity>
 
         <!-- ChromeTabbedActivity related -->
         <activity android:name="org.chromium.chrome.browser.ChromeTabbedActivity"
-             android:theme="@style/MainTheme"
+             android:theme="@style/TabbedModeTheme"
              android:exported="false"
-             android:windowBackground="@drawable/window_background"
              android:windowSoftInputMode="adjustResize"
              android:launchMode="singleTask"
              android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize"
              android:hardwareAccelerated="false">
         </activity>
         <activity android:name="org.chromium.chrome.browser.ChromeTabbedActivity2"
-             android:theme="@style/MainTheme"
+             android:theme="@style/TabbedModeTheme"
              android:exported="false"
-             android:windowBackground="@drawable/window_background"
              android:windowSoftInputMode="adjustResize"
              android:taskAffinity="{{ manifest_package }}.ChromeTabbedActivity2"
              android:launchMode="singleTask"
              android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize"
              android:hardwareAccelerated="false">
         </activity>
         <activity android:name="org.chromium.chrome.browser.multiwindow.MultiInstanceChromeTabbedActivity"
-            android:theme="@style/MainTheme"
+            android:theme="@style/TabbedModeTheme"
             android:exported="false"
-            android:windowBackground="@drawable/window_background"
             android:windowSoftInputMode="adjustResize"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize"
             android:hardwareAccelerated="false">
         </activity>
 
         <activity android:name="org.chromium.chrome.browser.sync.ui.PassphraseActivity"
             android:theme="@style/MainTheme"
             android:autoRemoveFromRecents="true">
         </activity>
         <activity android:name="org.chromium.chrome.browser.firstrun.FirstRunActivity"
@@ skipping 529 matching lines @@
              choose the first MediaRouteController that can play it, so the order of the list can be important.
              The most specific MediaRouteControllers should be listed first, followed by more generic ones.
              The downstream manifest replaces this block, and hence replaces the list of media route
              controllers with its own list. -->
         <meta-data android:name="org.chromium.content.browser.REMOTE_MEDIA_PLAYERS"
             android:value="org.chromium.chrome.browser.media.remote.DefaultMediaRouteController"/>
 
         {% endblock %}
     </application>
 </manifest>