Add D-Bus method to get the duration since an update completed.

This new API can be used by Chrome to automatically reboot the device
at N hours after updating. This is implemented as a D-Bus method that
returns the number of micro-seconds on the wall-clock since the update
completed. If the device has not updated, the D-Bus method returns an
error.

For robustness, durations are measured using the CLOCK_BOOTTIME clock
instead of the usual CLOCK_REALTIME clock. This avoids interference
with NTP adjustments, the RTC clock being wrong and other things.

BUG=chromium:218192
TEST=New unit test + unit tests pass + manual test on a device using
    the gdbus(1) command as the chronos user.

Change-Id: I51d44d69afe2d3024bb0780916c3c4e3f8ebb19e
Reviewed-on: https://chromium-review.googlesource.com/173032
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/dbus_service.h b/dbus_service.h
index 6d449df..5d0f500 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -128,6 +128,14 @@
     gboolean* allowed,
     GError **error);
 
+// Returns the duration since the last successful update, as the
+// duration on the wallclock. Returns an error if the device has not
+// updated.
+gboolean update_engine_service_get_duration_since_update(
+    UpdateEngineService* self,
+    gint64* out_usec_wallclock,
+    GError **error);
+
 gboolean update_engine_service_emit_status_update(
     UpdateEngineService* self,
     gint64 last_checked_time,