Add high contrast keyboard shortcut to keyboard overlay

chrome/app/chromeos_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Chrome OS-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 4831 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

[stevenjb, 2017/02/16 19:45:13]

Please double check this with UX (I would expect just 'High contrast mode')

[afakhry, 2017/02/16 21:07:44]

This is how dmazzoni wrote it when the shortcut used to be Ctrl+Alt+H, but that
was reverted 5 months ago.

+  </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 1867 matching lines @@
     Attempts left: $1
   </message>
   <message name="IDS_REQUEST_PIN_DIALOG_PIN" desc="The Provider Identification Number abbreviation">
     PIN
   </message>
   <message name="IDS_REQUEST_PIN_DIALOG_PUK" desc="The Personal Unlocking Key (as used in mobile phones) abbreviation">
     PUK
   </message>
 
 </grit-part>