update_engine: Don't require reboot after install.

We don't require a reboot after installing a DLC. Instead just set
Update Engine's status to idle.

Resubmitting now that crbug.com/905704 is fixed.

BUG=chromium:907205,chromium:907632
TEST=Attempt a DLC install, UE no longer prompts for a reboot.

(cherry picked from commit d772aaebdbd0c53d3e46e21df5783d437809108a)

Change-Id: I061ee8d6268a922ae21f310211acea5b645a3ebb
Reviewed-on: https://chromium-review.googlesource.com/1362572
Commit-Ready: Colin Howes <chowes@google.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_attempter.cc b/update_attempter.cc
index 50c72f0..179b455 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -1051,6 +1051,13 @@
     system_state_->payload_state()->SetStagingWaitPeriod(TimeDelta());
     prefs_->Delete(kPrefsUpdateFirstSeenAt);
 
+    if (is_install_) {
+      LOG(INFO) << "DLC successfully installed, no reboot needed.";
+      SetStatusAndNotify(UpdateStatus::IDLE);
+      ScheduleUpdates();
+      return;
+    }
+
     SetStatusAndNotify(UpdateStatus::UPDATED_NEED_REBOOT);
     ScheduleUpdates();
     LOG(INFO) << "Update successfully applied, waiting to reboot.";