MD Settings: Add remapping options for Backspace and Escape

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- About Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_ABOUT_PAGE_DETAILED_BUILD_INFO" desc="Label describing the section that shows detailed information about the current Chrome build.">
       Detailed build information
     </message>
     <message name="IDS_SETTINGS_ABOUT_PAGE_ARC_VERSION" desc="Label for ARC version">
       ARC Version
@@ skipping 2348 matching lines @@
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_KEY_CAPS_LOCK" desc="In Device Settings, the label and dropdown list item for the Caps Lock key.">
       Caps Lock
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_KEY_DIAMOND" desc="In Device Settings, the label for the Diamond key.">
       Diamond
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_KEY_ESCAPE" desc="In Device Settings, the dropdown list item for the Escape key.">
       Escape
     </message>
+    <message name="IDS_SETTINGS_KEYBOARD_KEY_BACKSPACE" desc="In Device Settings, the dropdown list item for the Backspace key.">
+      Backspace
+    </message>
     <message name="IDS_SETTINGS_KEYBOARD_KEY_DISABLED" desc="In Device Settings, the dropdown list item for a disabled key.">
       Disabled
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_SEND_FUNCTION_KEYS" desc="In Device Settings, the checkbox label for interpreting the top-row keys as function keys instead.">
       Treat top-row keys as function keys
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_SEND_FUNCTION_KEYS_DESCRIPTION" desc="In Device Settings, the label describing how to use the top-row keys' original actions when they are set to behave like function keys.">
       Hold the Search key to switch the behavior of the top-row keys.
     </message>
     <message name="IDS_SETTINGS_KEYBOARD_AUTO_REPEAT_ENABLE" desc="The checkbox label for enabling keyboard auto-repeat.">
@@ skipping 172 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>