system_api: battery_percent default in PowerSupplyProperties

Update powerd's PowerSupplyProperties protocol buffer to
specify a default of -1.0 for its battery_percent field.
powerd has historically set this field to -1 when a battery
isn't present, but it's probably more correct to leave
battery-related fields unset in this case. Chrome reads
these fields unconditionally in some cases, though, so we
want to maintain the same user-facing behavior.

BUG=chromium:724903
TEST=none

Change-Id: I7cf0b9e46e7e90742185c7785a9b2133cd6af1ff
Reviewed-on: https://chromium-review.googlesource.com/598512
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
1 file changed