Actually this is a regression by https://crrev.com/c/1112967.

Looking into it.

+yhanada@ who said he'll revert the CL in question.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/421412e7cce9db9512c8e4b816323aa0eed0def5

commit 421412e7cce9db9512c8e4b816323aa0eed0def5
Author: Yuichiro Hanada <yhanada@chromium.org>
Date: Fri Jul 13 09:07:50 2018

Send synthetic key events to ARC++ apps.

crrev.com/c/1112967 adds a check that sending key events is native or
not and synthetic key events were not sent to exo clients after the CL.
The check was introduced to make sure key press event and key release
event always have the same key code. It's needed for Crostini apps
tracking pressed key sets by observing key press/release events.
However, after the CL, all synthetic events (for example, the back
button on the shelf or keyboard navigation when ChromeVox is enabled)
were not sent to ARC++ apps anymore.
This CL is a quick hack for fixing this problem on ARC++ apps. We have
to find a cleaner way to fix all problems later.

Bug:  859071 ,  859797 ,  847500 ,  859797 ,  862140 
Test: Confirmed the back button on the shelf works on ARC++ apps.
Change-Id: Ia1da441cca8bc43687327b69cccf16fb8692046b
Reviewed-on: https://chromium-review.googlesource.com/1133102
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574866}
[modify] https://crrev.com/421412e7cce9db9512c8e4b816323aa0eed0def5/components/exo/keyboard.cc
[modify] https://crrev.com/421412e7cce9db9512c8e4b816323aa0eed0def5/components/exo/keyboard.h
[modify] https://crrev.com/421412e7cce9db9512c8e4b816323aa0eed0def5/components/exo/notification_surface.cc