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

chromeos/network/client_cert_resolver_unittest.cc

Index: chromeos/network/client_cert_resolver_unittest.cc
diff --git a/chromeos/network/client_cert_resolver_unittest.cc b/chromeos/network/client_cert_resolver_unittest.cc
index db41cb8e4b36e99a126766fa7d0ffb216c1abb40..021744b4bc294fc760094ad4a13cc31c3104eee7 100644
--- a/chromeos/network/client_cert_resolver_unittest.cc
+++ b/chromeos/network/client_cert_resolver_unittest.cc
@@ -65,6 +65,7 @@ class ClientCertResolverTest : 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_ =
@@ -79,7 +80,6 @@ class ClientCertResolverTest : 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());
   }