Disable Set wallpaper option in context menu for guest user instead of remove it

ash/ash_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
 This file contains the strings for ash.
 -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/ash_strings.h" type="rc_header">
@@ skipping 157 matching lines @@
         Applications
       </message>
       <message name="IDS_AURA_NEW_TAB" desc="The text label of the New Tab menu item">
         New tab
       </message>
       <message name="IDS_AURA_LAUNCHER_OVERFLOW_NAME" desc="The title used for the Aura overflow button in the launcher">
         Overflow Button
       </message>
 
       <message name="IDS_AURA_SET_DESKTOP_WALLPAPER" desc="The label used for change wallpaper in context menu">
-        Set Wallpaper...
+        Set wallpaper...
       </message>
 
       <message name="IDS_FRAME_MAXIMIZE_BUTTON_TOOLTIP" desc="Tooltip for the maximize/restore button in window frames in aura">
         Pull down to minimize, left or right to tile
       </message>
 
       <!-- Status tray items -->
       <message name="IDS_ASH_STATUS_TRAY_ACCESSIBLE_NAME" desc="The accessible name of the status tray.">
         Status tray
       </message>
@@ skipping 123 matching lines @@
       <message name="IDS_ASH_SECONDARY_MONITOR_SHORTCUT" desc="The shortcut text on the secondary monitors.">
         Ctrl + 
       </message>
       <message name="IDS_ASH_STATUS_TRAY_PREVIOUS_MENU" desc="The accessible text for header entries for detailed versions of status tray items.">
         Previous menu
       </message>
     </messages>
   </release>
 </grit>