Add a SigninAllowed policy.

chrome/app/chromium_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are distribution specific.
 These strings will not be translated and only english versions will
 be available for now. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/chromium_strings.h" type="rc_header">
       <emit emit_type='prepend'></emit>
@@ skipping 637 matching lines @@
         <message name="IDS_CREATE_SHORTCUTS_ERROR_LABEL" desc="Contents of the main label in the error dialog box when creating an application shortcut failed.">
           Chromium cannot create an application shortcut because it cannot determine the current desktop environment.
         </message>
       </if>
 
       <!-- Autolaunch infobar -->
       <message name="IDS_AUTO_LAUNCH_INFOBAR_TEXT" desc="The text to show in the infobar when Chromium was automatically launched on startup">
         Chromium is configured to automatically launch when you start your computer.
       </message>
 
+      <!-- Managed user settings page -->
+      <message name="IDS_SIGNIN_SYNC_ALLOWED" desc="A checkbox in the Managed User Settings dialog for enabling sign in and sync.">
+        Allow signing into Chromium.
+      </message>
+
       <!-- about:flags -->
       <if expr="not pp_ifdef('chromeos')">
         <message name="IDS_FLAGS_RELAUNCH_NOTICE" desc="Notifies the user that he needs to relaunch Chromium. Shown next to a button that says 'Relaunch Now'.">
           Your changes will take effect the next time you relaunch Chromium.
         </message>
       </if>
       <if expr="pp_ifdef('chromeos')">
         <message name="IDS_FLAGS_RELAUNCH_NOTICE" desc="Notifies the user that he needs to restart Chromium OS. Shown next to a button that says 'Restart Now'.">
           Your changes will take effect the next time you restart your device.
         </message>
@@ skipping 293 matching lines @@
       <!-- App list -->
       <message name="IDS_APP_LIST_SHORTCUT_NAME" desc="Name for the Chromium App List to appear in the taskbar and in any shortcuts to it.">
         Chromium App Launcher
       </message>
       <message name="IDS_APP_LIST_OPT_IN_TEXT" desc="Text to appear in the prompt to use the Chrome app list.">
         Use the Chromium App Launcher to launch your apps.
       </message>
     </messages>
   </release>
 </grit>