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/fake_clock.h b/fake_clock.h
index 105d664..d920aaf 100644
--- a/fake_clock.h
+++ b/fake_clock.h
@@ -22,6 +22,10 @@
     return monotonic_time_;
   }
 
+  virtual base::Time GetBootTime() {
+    return boot_time_;
+  }
+
   void SetWallclockTime(const base::Time &time) {
     wallclock_time_ = time;
   }
@@ -30,9 +34,14 @@
     monotonic_time_ = time;
   }
 
+  void SetBootTime(const base::Time &time) {
+    boot_time_ = time;
+  }
+
  private:
   base::Time wallclock_time_;
   base::Time monotonic_time_;
+  base::Time boot_time_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeClock);
 };