@afakhry could you please add "Toggle high contrast" to the overlay? Thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11706d336c17042bfaa75d017a124b85647d4c11

commit 11706d336c17042bfaa75d017a124b85647d4c11
Author: afakhry <afakhry@chromium.org>
Date: Fri Feb 17 02:24:11 2017

Add high contrast keyboard shortcut to keyboard overlay

This CL adds the Search+Shift+H keyboard shortcut (which toggles the
high contrast mode) to the keyboard overlay.

BUG= 676510 
TEST=manually, open keyboard overlay (Ctrl+Alt+/), and press and hold
Search and Shift. The key H should show "Toggle High Contrast Mode".
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2699643004
Cr-Commit-Position: refs/heads/master@{#451186}

[modify] https://crrev.com/11706d336c17042bfaa75d017a124b85647d4c11/chrome/app/chromeos_strings.grdp
[modify] https://crrev.com/11706d336c17042bfaa75d017a124b85647d4c11/chrome/browser/resources/chromeos/keyboard_overlay_data.js
[modify] https://crrev.com/11706d336c17042bfaa75d017a124b85647d4c11/chrome/browser/ui/webui/chromeos/keyboard_overlay_ui.cc

verified on 58.0.3029.51 / 9334.33.0 build