update_engine: Add rollback_allowed policy in Omaha requests

Send rollback_allowed="true" parameter to Omaha if rollback is
allowed through enterprise policy.

This parameter indicates to Omaha that the returned image might
be a rollback image, if the targetversionprefix parameter is set
and it is an earlier version than the one which is currently on
the device.

Also, this CL changes update_engine behavior to only send
targetversionprefix to Omaha if it's not empty.

BUG=chromium:814830
TEST='cros_run_unit_tests --board=cyan --packages update_engine'

Change-Id: Iaa5503e217631311fb9ba3a5237ccbfedf04afe9
Reviewed-on: https://chromium-review.googlesource.com/932123
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_manager/chromeos_policy_unittest.cc b/update_manager/chromeos_policy_unittest.cc
index 9538c4e..c7380e8 100644
--- a/update_manager/chromeos_policy_unittest.cc
+++ b/update_manager/chromeos_policy_unittest.cc
@@ -189,9 +189,6 @@
   // Override specific device policy attributes.
   fake_state_.device_policy_provider()->var_target_version_prefix()->
       reset(new string("1.2"));
-  fake_state_.device_policy_provider()->var_rollback_to_target_version()->reset(
-      new RollbackToTargetVersion(
-          RollbackToTargetVersion::kRollbackWithFullPowerwash));
   fake_state_.device_policy_provider()
       ->var_rollback_allowed_milestones()
       ->reset(new int(5));
@@ -205,13 +202,70 @@
                      &Policy::UpdateCheckAllowed, &result);
   EXPECT_TRUE(result.updates_enabled);
   EXPECT_EQ("1.2", result.target_version_prefix);
-  EXPECT_EQ(RollbackToTargetVersion::kRollbackWithFullPowerwash,
-            result.rollback_to_target_version);
   EXPECT_EQ(5, result.rollback_allowed_milestones);
   EXPECT_EQ("foo-channel", result.target_channel);
   EXPECT_FALSE(result.is_interactive);
 }
 
+TEST_F(UmChromeOSPolicyTest, UpdateCheckAllowedRollbackAllowed) {
+  // Update check is allowed, response includes attributes for use in the
+  // request.
+  SetUpdateCheckAllowed(true);
+
+  // Override RollbackToTargetVersion device policy attribute.
+  fake_state_.device_policy_provider()->var_rollback_to_target_version()->reset(
+      new RollbackToTargetVersion(
+          RollbackToTargetVersion::kRollbackWithFullPowerwash));
+
+  UpdateCheckParams result;
+  ExpectPolicyStatus(
+      EvalStatus::kSucceeded, &Policy::UpdateCheckAllowed, &result);
+  EXPECT_TRUE(result.rollback_allowed);
+}
+
+TEST_F(UmChromeOSPolicyTest, UpdateCheckAllowedRollbackDisabled) {
+  // Update check is allowed, response includes attributes for use in the
+  // request.
+  SetUpdateCheckAllowed(true);
+
+  // Override RollbackToTargetVersion device policy attribute.
+  fake_state_.device_policy_provider()->var_rollback_to_target_version()->reset(
+      new RollbackToTargetVersion(RollbackToTargetVersion::kDisabled));
+
+  UpdateCheckParams result;
+  ExpectPolicyStatus(
+      EvalStatus::kSucceeded, &Policy::UpdateCheckAllowed, &result);
+  EXPECT_FALSE(result.rollback_allowed);
+}
+
+TEST_F(UmChromeOSPolicyTest, UpdateCheckAllowedRollbackUnspecified) {
+  // Update check is allowed, response includes attributes for use in the
+  // request.
+  SetUpdateCheckAllowed(true);
+
+  // Override RollbackToTargetVersion device policy attribute.
+  fake_state_.device_policy_provider()->var_rollback_to_target_version()->reset(
+      new RollbackToTargetVersion(RollbackToTargetVersion::kUnspecified));
+
+  UpdateCheckParams result;
+  ExpectPolicyStatus(
+      EvalStatus::kSucceeded, &Policy::UpdateCheckAllowed, &result);
+  EXPECT_FALSE(result.rollback_allowed);
+}
+
+TEST_F(UmChromeOSPolicyTest, UpdateCheckAllowedRollbackNotSet) {
+  // Update check is allowed, response includes attributes for use in the
+  // request.
+  SetUpdateCheckAllowed(true);
+
+  // Don't set RollbackToTargetVersion device policy attribute.
+
+  UpdateCheckParams result;
+  ExpectPolicyStatus(
+      EvalStatus::kSucceeded, &Policy::UpdateCheckAllowed, &result);
+  EXPECT_FALSE(result.rollback_allowed);
+}
+
 TEST_F(UmChromeOSPolicyTest,
        UpdateCheckAllowedUpdatesDisabledForUnofficialBuilds) {
   // UpdateCheckAllowed should return kAskMeAgainLater if this is an unofficial