Fix KioskRequiredPlatformVersion unittests.

The unittests expect calling GetKioskAppRequiredPlatformVersion which
only happens if USE_LIBCROS.

Test: GTEST_FILTER="*Kiosk*" ./update_engine_unittests
Bug: 26955860

Change-Id: If06b61bbe75d3d99dd7a6353ec4629d8df1c2e15
diff --git a/update_manager/real_system_provider_unittest.cc b/update_manager/real_system_provider_unittest.cc
index a6ff2d9..112eba5 100644
--- a/update_manager/real_system_provider_unittest.cc
+++ b/update_manager/real_system_provider_unittest.cc
@@ -81,6 +81,7 @@
   UmTestUtils::ExpectVariableHasValue(false, provider_->var_is_oobe_complete());
 }
 
+#if USE_LIBCROS
 TEST_F(UmRealSystemProviderTest, KioskRequiredPlatformVersion) {
   const std::string kRequiredPlatformVersion("1234.0.0");
   EXPECT_CALL(*service_interface_mock_,
@@ -101,5 +102,6 @@
   UmTestUtils::ExpectVariableHasValue(
       std::string(), provider_->var_kiosk_required_platform_version());
 }
+#endif
 
 }  // namespace chromeos_update_manager