Introduce StickyKeys settings item with behind flag.

chrome/app/chromeos_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- ChromeOS-specific strings (included from generated_resources.grd).
      Everything in this file is wrapped in <if expr="pp_ifdef('chromeos')">. -->
 <grit-part>
   <message name="IDS_EMPTY_STRING" desc="Empty string, exist only to make code generic. No translation required.">
   </message>
   <!-- Idle logout dialog for retail mode -->
   <message name="IDS_IDLE_LOGOUT_TITLE" desc="Dialog title for the idle logout dialog. Used in retail mode only.">
     Are you still there?
   </message>
@@ skipping 816 matching lines @@
   </message>
   <message name="IDS_OPTIONS_SETTINGS_BLUETOOTH_FORGET_FAILED">
     Failed to disconnect and unpair from "<ph name="DEVICE_NAME">%1<ex>Nexus S</ex></ph>".
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_EXPLANATION" desc="In the settings tab, this text explains the accessibiliy features.">
     Enable accessibility features to make your device easier to use.
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_LARGE_CURSOR_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for the large cursor.">
     Enable Large Cursor
   </message>
+  <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_STICKY_KEYS_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for the sticky keys.">
+    Enable Sticky Keys
+  </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_SPOKEN_FEEDBACK_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for spoken feedback.">
     Enable spoken feedback
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_HIGH_CONTRAST_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for high contrast mode.">
     Enable high contrast
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_SCREEN_MAGNIFIER_DESCRIPTION" desc="In the settings tab, the text next to the dropdown for screen magnifier.">
     Enable screen magnifier
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_SCREEN_MAGNIFIER_OFF" desc="In the settings tab, the option indication the screen magnifier is off.">
@@ skipping 2849 matching lines @@
     You are currently connected to the <ph name="NETWORK_TYPE">$1<ex>cellular</ex></ph> network.
   </message>
   <message name="IDS_UPGRADE_NETWORK_LIST_CELLULAR_ALLOWED" desc="Status label: for updates you need to connect to an Ethernet or Wi-Fi">
     To check for updates, please use Ethernet or Wi-Fi.
   </message>
   <message name="IDS_UPGRADE_NETWORK_LIST_CELLULAR_DISALLOWED" desc="Status label: for updates you need to connect to an Ethernet, mobile data or Wi-Fi">
     To check for updates, please use Ethernet, Wi-Fi or mobile data.
   </message>
 
 </grit-part>