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 3f9460784dc804789221989b7aac0ca73faa9e2d..0aa48e4d9bf055920e3035d12ac60eae592569d8 100644
--- a/chromeos/network/network_cert_migrator_unittest.cc
+++ b/chromeos/network/network_cert_migrator_unittest.cc
@@ -52,6 +52,7 @@ 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_ =
@@ -62,7 +63,6 @@ class NetworkCertMigratorTest : public testing::Test {
 
     CertLoader::Initialize();
     CertLoader* cert_loader_ = CertLoader::Get();
-    cert_loader_->SetSlowTaskRunnerForTest(message_loop_.message_loop_proxy());
     cert_loader_->StartWithNSSDB(test_nssdb_.get());
   }