hterm: add chromeos ctrl+alt+z toggle chromevox as os default binding
hterm: add chromeos ctrl+alt+z toggle chromevox as os default binding

Bug: 1086322
Change-Id: Ibf9496aad97f93319d90d71397601355321ce438
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2224414
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm