MD Settings: Display: Add unified desktop control and modify api

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- Main Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_SECONDARY_USER_BANNER" desc="Banner displayed in settings page when the user is secondary in a multi-profile session.">
       Some settings belonging to <ph name="PRIMARY_EMAIL">$1<ex>john@google.com</ex></ph> are being shared with you. These settings only affect your account when using multiple sign-in.
     </message>
   </if>
 
@@ skipping 2810 matching lines @@
     </message>
     <message name="IDS_SETTINGS_DISPLAY_ARRANGEMENT_TITLE" desc="In Device Settings > Displays, the label for the display arrangement section.">
       Arrangement
     </message>
     <message name="IDS_SETTINGS_DISPLAY_ARRANGEMENT_TEXT" desc="In Device Settings > Displays, text describing to use a mouse to drag the displays and change the arrangement.">
       Drag to move a display
     </message>
     <message name="IDS_SETTINGS_DISPLAY_MIRROR" desc="In Device Settings > Displays, the label for the control for mirroring.">
       Mirroring
     </message>
+    <message name="IDS_SETTINGS_DISPLAY_UNIFIED_DESKTOP" desc="In Device Settings > Displays, the label for the control for the unified desktop feature.">
+      Allow windows to span displays
+    </message>
     <message name="IDS_SETTINGS_DISPLAY_SCREEN" desc="In Device Settings > Displays, the label for the dropdown menu for changing the dispay type.">
       Screen
     </message>
     <message name="IDS_SETTINGS_DISPLAY_SCREEN_PRIMARY" desc="The label for the dropdown menu list item to set the selected screen as primary display.">
       Primary display
     </message>
     <message name="IDS_SETTINGS_DISPLAY_SCREEN_EXTENDED" desc="The label for the dropdown menu list item to set the selected screen as extended display.">
       Extended display
     </message>
     <message name="IDS_SETTINGS_DISPLAY_RESOLUTION_TITLE" desc="In Device Settings > Displays, the label for the section for changing a display's resolution.">
@@ skipping 134 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_BUTTON" desc="Text for the button that changes a user's network proxy settings.">
       Change proxy settings
     </message>
   </if>
 </grit-part>