power: Handle reason in RequestRestart method calls.

Read an optional enum in D-Bus calls to the RequestRestart
method describing the reason for the restart (currently,
either a user request or a system update).

BUG=chromium:372074
TEST=applied an update and checked that the restart reason
     was logged correctly in powerd's log
CQ-DEPEND=I73f67cca9875b00bd944e4ff9779ad24fc4cd738

Change-Id: Ib6b13e104970c9a8d012309949574a8f667d2d8c
Reviewed-on: https://chromium-review.googlesource.com/206072
Commit-Queue: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
3 files changed