The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/system_api/+/99669d7f1774875c2446e445681dc69754c5a847

commit 99669d7f1774875c2446e445681dc69754c5a847
Author: Daniel Erat <derat@chromium.org>
Date: Thu Jun 28 05:07:03 2018

system_api: Add GetKeyboardBrightnessPercent constant.

Add a constant for a new GetKeyboardBrightnessPercent method
that will be exported by powerd.

BUG= chromium:850807 
TEST=built it

Change-Id: I4ad360e01c9d433b75e7c28260f8a139ec47933c
Reviewed-on: https://chromium-review.googlesource.com/1112820
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Abhishek Bhardwaj <abhishekbh@chromium.org>

[modify] https://crrev.com/99669d7f1774875c2446e445681dc69754c5a847/dbus/power_manager/dbus-constants.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/c250b195399fe1a29ff37592c30400438932160b

commit c250b195399fe1a29ff37592c30400438932160b
Author: Daniel Erat <derat@chromium.org>
Date: Thu Jun 28 05:07:04 2018

power: Export GetKeyboardBrightnessPercent D-Bus method.

Make powerd export a GetKeyboardBrightnessPercent D-Bus
method that returns a single double arg containing the
keyboard backlight brightness as a percentage in [0.0,
100.0]. This is similar to the existing
GetScreenBrightnessPercent method.

BUG= chromium:850807 
TEST=added unit tests; also manually verified by running:
     dbus-send --type=method_call --system --print-reply \
       --dest=org.chromium.PowerManager \
       /org/chromium/PowerManager \
       org.chromium.PowerManager.GetKeyboardBrightnessPercent
CQ-DEPEND=I4ad360e01c9d433b75e7c28260f8a139ec47933c

Change-Id: I9ef38d1aacf634a4c81b01be5d65745527728176
Reviewed-on: https://chromium-review.googlesource.com/1112854
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Abhishek Bhardwaj <abhishekbh@chromium.org>

[modify] https://crrev.com/c250b195399fe1a29ff37592c30400438932160b/power_manager/powerd/policy/keyboard_backlight_controller_unittest.cc
[modify] https://crrev.com/c250b195399fe1a29ff37592c30400438932160b/power_manager/powerd/policy/keyboard_backlight_controller.cc
[modify] https://crrev.com/c250b195399fe1a29ff37592c30400438932160b/power_manager/powerd/policy/keyboard_backlight_controller.h

Nikhil is looking into updating Chrome's PowerManagerClient class to support calling this method. I'm unable to assign ownership, though (see http://g/chromeos-chatty-eng/0lv6jt5_sW0 for the fix, I think).

https://bugs.chromium.org/p/chromium/issues/detail?id=850807 fixes this.