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 918 matching lines @@
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_TOUCHPAD_TAP_DRAGGING_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for tap dragging.">
     Enable tap dragging
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_VIRTUAL_KEYBOARD_DESCRIPTION" desc="In the settings tab, the text next to the checkbox for virtual keyboard.">
     Enable virtual keyboard
   </message>
   <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_SHOULD_ALWAYS_SHOW_MENU" desc="In the settings tab, the text next to the checkbox to show the accessibility tray menu regardless of the state of a11y features.">
     Show accessibility options in the system menu
   </message>
+  <message name="IDS_OPTIONS_SETTINGS_ACCESSIBILITY_STICKY_KEYS_DESCRIPTION" desc="Inthe settings tab, the text next to the checkbox for the sticky keys.">
+    Enable sticky keys (to perform keyboard shortcuts by typing them sequentially)
+  </message>
   <message name="IDS_OPTIONS_SETTINGS_DISPLAY_OPTIONS_BUTTON_LABEL" desc="Label for the button opening display settings tab.">
     Display settings
   </message>
   <message name="IDS_OPTIONS_SETTINGS_DISPLAY_OPTIONS_TAB_TITLE" desc="Title for the display settings tab.">
     Manage displays
   </message>
   <message name="IDS_OPTIONS_SETTINGS_DISPLAY_OPTIONS_OPTIONS" desc="The section title for general options for a selected display.">
     Options:
   </message>
   <message name="IDS_OPTIONS_SETTINGS_DISPLAY_OPTIONS_RESOLUTION" desc="The section title for resolution option for a selected display.">
@@ skipping 2800 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>