I think I might end up putting the ambient light path flag in backlight_tool rather than in check_powerd_config, since it involves doing more work than just checking powerd's config.

So the commands would be:

backlight_tool --get_ambient_light_path
check_powerd_config --internal_backlight_ambient_light_steps

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850

commit 1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850
Author: Daniel Erat <derat@chromium.org>
Date: Thu Aug 09 03:46:00 2018

power: Add --get_ambient_light_path to backlight_tool.

Make the backlight_tool helper support a new
--get_ambient_light_path that instructs it to print the
ambient light sensor illuminance file that powerd would
monitor.

Also update some old code to take base::FilePath arguments
instead of std::string.

BUG= chromium:868158 
TEST=manual: ran "backlight_tool --get_ambient_light_path"
     on a caroline device and got
     "/sys/bus/iio/devices/iio:device0/in_illuminance_raw"

Change-Id: I8faaa7a9dd3fe51eb413e894ba59095a020b843f
Reviewed-on: https://chromium-review.googlesource.com/1166240
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>

[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/ambient_light_sensor.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/ambient_light_sensor.h
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/tools/backlight_tool.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/peripheral_battery_watcher.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/async_file_reader.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/main.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/async_file_reader_unittest.cc
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/async_file_reader.h
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/peripheral_battery_watcher.h
[modify] https://crrev.com/1f9e5ad0aaad1c891dc0ab72b7e834973b8d9850/power_manager/powerd/system/ambient_light_sensor_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/d6703b11f08382f60698082a26553cb614e23991

commit d6703b11f08382f60698082a26553cb614e23991
Author: Daniel Erat <derat@chromium.org>
Date: Fri Aug 10 05:04:33 2018

power: Add --internal_backlight_ambient_light_steps.

Add --internal_backlight_ambient_light_steps to
check_powerd_config. This flag instructs the tool to print
the value of the internal_backlight_als_steps powerd pref to
stdout.

BUG= chromium:868158 
TEST=ran "check_powerd_config
     --internal_backlight_ambient_light_steps" on a caroline
     device and saw "80.0 63.0 -1 -1"

Change-Id: Ie90210e83e418f0bd532441db1fc46af43f4d002
Reviewed-on: https://chromium-review.googlesource.com/1169431
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>

[modify] https://crrev.com/d6703b11f08382f60698082a26553cb614e23991/power_manager/tools/check_powerd_config.cc