Make NSSCertDatabase::ListCerts work async on a worker thread.

chromeos/network/network_cert_migrator_unittest.cc

Index: chromeos/network/network_cert_migrator_unittest.cc
diff --git a/chromeos/network/network_cert_migrator_unittest.cc b/chromeos/network/network_cert_migrator_unittest.cc
index 0f2f56c6773ad057e5d7424a6927c3cbe5590ca3..3f9cdc0b2adbf6b009c7d15fc0e4d5037e44781b 100644
--- a/chromeos/network/network_cert_migrator_unittest.cc
+++ b/chromeos/network/network_cert_migrator_unittest.cc
@@ -53,6 +53,7 @@ class NetworkCertMigratorTest : public testing::Test {
         crypto::GetPrivateSlotForChromeOSUser(
             user_.username_hash(),
             base::Callback<void(crypto::ScopedPK11Slot)>())));
+    test_nssdb_->SetSlowTaskRunnerForTest(message_loop_.message_loop_proxy());
 
     DBusThreadManager::InitializeWithStub();
     service_test_ =
@@ -63,7 +64,6 @@ class NetworkCertMigratorTest : public testing::Test {
 
     CertLoader::Initialize();
     CertLoader* cert_loader_ = CertLoader::Get();
-    cert_loader_->SetSlowTaskRunnerForTest(message_loop_.message_loop_proxy());
     cert_loader_->set_hardware_backed_for_test();
     cert_loader_->StartWithNSSDB(test_nssdb_.get());
   }