update_engine: Force Update + Install uniform logic pattern

The change here is to make the exit points within |CheckForUpdate()| and
|CheckForInstall()| uniform with simpler logic.

When a force update or install is called, it is heavily dependent on the
|forced_update_pending_callback_| and the existence of a scheduled
update in the message loop. The |forced_update_pending_callback_| is the
one that notifies a change hence calling upon |OnUpdateScheduled()| that
was waiting on the message loop.

The call to |ScheduleUpdates()| before forcing the callback is to
guarantee that the |forced_update_pending_callback_| will take effect.

Even if the |forced_update_pending_callback_| is not set, it is not a
failure. Simply, the periodic check will use the set forced variables
when it is scheduled.

BUG=none
TEST=FEATURES="test" emerge-${BOARD} update_engine

Change-Id: Ic97671a70880c8b05d513a8dea93fce3b982bbf6
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index 203d704..b7d0997 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -100,6 +100,7 @@
   // Expects:
   string expected_forced_app_version = "";
   string expected_forced_omaha_url = "";
+  bool should_schedule_updates_be_called = true;
   bool expected_result = true;
 };
 
@@ -332,6 +333,8 @@
             attempter_.forced_app_version());
   EXPECT_EQ(cfu_params_.expected_forced_omaha_url,
             attempter_.forced_omaha_url());
+  EXPECT_EQ(cfu_params_.should_schedule_updates_be_called,
+            attempter_.WasScheduleUpdatesCalled());
 }
 
 void UpdateAttempterTest::ScheduleQuitMainLoop() {
@@ -1391,6 +1394,8 @@
   cfu_params_.status = UpdateStatus::CHECKING_FOR_UPDATE;
   // GIVEN a interactive update.
 
+  // THEN |ScheduleUpdates()| should not be called.
+  cfu_params_.should_schedule_updates_be_called = false;
   // THEN result should indicate failure.
   cfu_params_.expected_result = false;
 
@@ -1564,6 +1569,34 @@
   TestCheckForUpdate();
 }
 
+TEST_F(UpdateAttempterTest, CheckForUpdateMissingForcedCallback1) {
+  // GIVEN a official build.
+  // GIVEN forced callback is not set.
+  attempter_.set_forced_update_pending_callback(nullptr);
+
+  // THEN we except forced app version + forced omaha url to be cleared.
+  // THEN |ScheduleUpdates()| should not be called.
+  cfu_params_.should_schedule_updates_be_called = false;
+
+  TestCheckForUpdate();
+}
+
+TEST_F(UpdateAttempterTest, CheckForUpdateMissingForcedCallback2) {
+  // GIVEN a non offical build with dev features enabled.
+  cfu_params_.is_official_build = false;
+  cfu_params_.are_dev_features_enabled = true;
+  // GIVEN forced callback is not set.
+  attempter_.set_forced_update_pending_callback(nullptr);
+
+  // THEN the forced app version + forced omaha url changes based on input.
+  cfu_params_.expected_forced_app_version = cfu_params_.app_version;
+  cfu_params_.expected_forced_omaha_url = cfu_params_.omaha_url;
+  // THEN |ScheduleUpdates()| should not be called.
+  cfu_params_.should_schedule_updates_be_called = false;
+
+  TestCheckForUpdate();
+}
+
 TEST_F(UpdateAttempterTest, CheckForInstallTest) {
   fake_system_state_.fake_hardware()->SetIsOfficialBuild(true);
   fake_system_state_.fake_hardware()->SetAreDevFeaturesEnabled(false);