Revert "update_engine: Don't require reboot after install."

This reverts commit d772aaebdbd0c53d3e46e21df5783d437809108a.

Reason for revert: crbug.com/905704

Original change's description:
> 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.
>
> BUG=chromium:907205,chromium:907632
> TEST=Attempt a DLC install, UE no longer prompts for a reboot.
>
> Change-Id: I4fba804ba8b62b8e781279c988aba0e1f21fe3cf
> Reviewed-on: https://chromium-review.googlesource.com/1344916
> Commit-Ready: Colin Howes <chowes@google.com>
> Tested-by: Colin Howes <chowes@google.com>
> Reviewed-by: Amin Hassani <ahassani@chromium.org>

TBR=norvez@chromium.org,ahassani@chromium.org,xiaochu@chromium.org,chromiumos-cl-exonerator@appspot.gserviceaccount.com,chowes@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:907205, chromium:907632
Change-Id: I4c95c7c0acacfb2a6589a7b2b82d445a8ef1f7d0
Reviewed-on: https://chromium-review.googlesource.com/1359108
Commit-Ready: Xiaochu Liu <xiaochu@chromium.org>
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
diff --git a/update_attempter.cc b/update_attempter.cc
index 7bb44cc..f47a63e 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -1049,13 +1049,6 @@
     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.";
diff --git a/update_attempter.h b/update_attempter.h
index 876d0fe..07a66b9 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -271,7 +271,6 @@
   FRIEND_TEST(UpdateAttempterTest, CreatePendingErrorEventResumedTest);
   FRIEND_TEST(UpdateAttempterTest, DisableDeltaUpdateIfNeededTest);
   FRIEND_TEST(UpdateAttempterTest, DownloadProgressAccumulationTest);
-  FRIEND_TEST(UpdateAttempterTest, InstallSetsStatusIdle);
   FRIEND_TEST(UpdateAttempterTest, MarkDeltaUpdateFailureTest);
   FRIEND_TEST(UpdateAttempterTest, PingOmahaTest);
   FRIEND_TEST(UpdateAttempterTest, ReportDailyMetrics);
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index 877396e..a395e1b 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -1216,17 +1216,6 @@
   EXPECT_EQ("", attempter_.forced_omaha_url());
 }
 
-TEST_F(UpdateAttempterTest, InstallSetsStatusIdle) {
-  attempter_.CheckForInstall({}, "http://foo.bar");
-  attempter_.status_ = UpdateStatus::DOWNLOADING;
-  EXPECT_TRUE(attempter_.is_install_);
-  attempter_.ProcessingDone(nullptr, ErrorCode::kSuccess);
-  UpdateEngineStatus status;
-  attempter_.GetStatus(&status);
-  // Should set status to idle after an install operation.
-  EXPECT_EQ(UpdateStatus::IDLE, status.status);
-}
-
 TEST_F(UpdateAttempterTest, TargetVersionPrefixSetAndReset) {
   attempter_.CalculateUpdateParams("", "", "", "1234", false, false, false);
   EXPECT_EQ("1234",