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/clock.h b/clock.h
index 027bcff..8cfe366 100644
--- a/clock.h
+++ b/clock.h
@@ -18,6 +18,8 @@
 
   virtual base::Time GetMonotonicTime();
 
+  virtual base::Time GetBootTime();
+
  private:
 
   DISALLOW_COPY_AND_ASSIGN(Clock);