Add the shortcut to toggle high contrast mode to the keyboard overlay

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="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 4642 matching lines @@
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_TOGGLE_CAPS_LOCK" desc="The text in the keyboard overlay to explain the shortcut (enable or disable caps lock).">
     Caps Lock
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_DISABLE_CAPS_LOCK" desc="The text in the keyboard overlay to explain the shortcut (disable caps lock when it's enabled).">
     Disable Caps Lock
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_TOGGLE_CHROMEVOX_SPOKEN_FEEDBACK" desc="The text in the keyboard overlay to explain the shortcut (enable or disable spoken feedback).">
     ChromeVox (spoken feedback)
   </message>
+  <message name="IDS_KEYBOARD_OVERLAY_TOGGLE_HIGH_CONTRAST_MODE" desc="The text in the keyboard overlay to explain the shortcut (Toggle high contrast mode).">
+    Toggle High Contrast Mode
+  </message>
   <message name="IDS_KEYBOARD_OVERLAY_TOGGLE_PROJECTION_TOUCH_HUD" desc="The text in the keyboard overlay to explain the shortcut.">
     Projection touch HUD
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_TOGGLE_SPEECH_INPUT" desc="The text in the keyboard overlay to explain the shortcut (start or stop speech input).">
     Speech input
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_UNDO" desc="The text in the keyboard overlay to explain the shortcut.">
     Undo
   </message>
   <message name="IDS_KEYBOARD_OVERLAY_VIEW_KEYBOARD_OVERLAY" desc="The text in the keyboard overlay to explain the shortcut.">
@@ skipping 1674 matching lines @@
   <message name="IDS_FLAGS_ENABLE_EHV_INPUT_NAME" desc="Name of the about: flag for enabling emoji, handwriting and voice input on opt-in IME menu.">
     Emoji, handwriting and voice input on opt-in IME menu
     </message>
   <message name="IDS_FLAGS_ENABLE_EHV_INPUT_DESCRIPTION" desc="Description of the about: flag enabling emoji, handwriting and voice input on opt-in IME menu.">
     Enable access to emoji, handwriting and voice input form opt-in IME menu.
   </message>
   <message name="IDS_HATS_DONE_BUTTON_LABEL" desc="Label used for the done button at the end of survey">
     Done
   </message>
 </grit-part>