[Android backup] Restore nothing if signed in user isn't valid

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 84 matching lines @@
     {% block extra_keyset_definitions %}
     {% endblock %}
 
     <!-- Set android:largeHeap to "true" to allow more than the default
          Java heap limit (32Mb on Nexus S, 48Mb on Xoom). -->
     <application android:name="{% block application_name %}org.chromium.chrome.browser.ChromeApplication{% endblock %}"
         android:icon="@mipmap/app_icon"
         android:label="@string/app_name"
         android:largeHeap="false"
         android:allowBackup="true"
+        android:fullBackupContent="@xml/chromebackupscheme"
+        {% block android_backup_agent %}
         android:backupAgent="org.chromium.chrome.browser.ChromeBackupAgent"
-        android:fullBackupContent="@xml/chromebackupscheme"
+        {% endblock %}
         android:supportsRtl="true"
         {% block extra_application_attributes %}{% endblock %}>
 
         <!-- Samsung MultiWindow Support -->
         <meta-data android:name="com.samsung.android.sdk.multiwindow.enable"
             android:value="true" />
         <meta-data android:name="com.samsung.android.sdk.multiwindow.multiinstance.enable"
             android:value="true" />
         <meta-data android:name="com.samsung.android.sdk.multiwindow.multiinstance.launchmode"
             android:value="singleTask" />
@@ skipping 707 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>