powerd.conf checks for existence of backlight files
powerd.conf checks for existence of backlight files

powerd.conf chowns the backlight files only if they exist.  This
allows the use of wildcards.

BUG=chromium-os:9631
TEST=Verified that backlight files are correctly changed on system where
they exist.  Verified that powerd starts correctly on system w/o them.

Change-Id: I2f4191b4d0e2469d08816dc8e4e32735aa080b0e
Signed-off-by: Simon Que <sque@chromium.org>

Review URL: http://codereview.chromium.org/5544006

joshfire/chromiumos-platform-init