Revert of Use TaskScheduler instead of WorkerPool in nss_cert_database.cc.

net/cert/nss_cert_database.h

Index: net/cert/nss_cert_database.h
diff --git a/net/cert/nss_cert_database.h b/net/cert/nss_cert_database.h
index d096f18d654ec9d1f48c08ad373a896c55b4d2b3..a9f5c8ac51035a0e7490a22059b76f1df8d8d234 100644
--- a/net/cert/nss_cert_database.h
+++ b/net/cert/nss_cert_database.h
@@ -25,6 +25,7 @@
 namespace base {
 template <class ObserverType>
 class ObserverListThreadSafe;
+class TaskRunner;
 }
 
 namespace net {
@@ -235,6 +236,10 @@ class NET_EXPORT NSSCertDatabase {
   // Check whether cert is stored in a hardware slot.
   bool IsHardwareBacked(const X509Certificate* cert) const;
 
+  // Overrides task runner that's used for running slow tasks.
+  void SetSlowTaskRunnerForTest(
+      const scoped_refptr<base::TaskRunner>& task_runner);
+
  protected:
   // Certificate listing implementation used by |ListCerts*| and
   // |ListCertsSync|. Static so it may safely be used on the worker thread.
@@ -243,6 +248,11 @@ class NET_EXPORT NSSCertDatabase {
   static void ListCertsImpl(crypto::ScopedPK11Slot slot,
                             CertificateList* certs);
 
+  // Gets task runner that should be used for slow tasks like certificate
+  // listing. Defaults to a base::WorkerPool runner, but may be overriden
+  // in tests (see SetSlowTaskRunnerForTest).
+  scoped_refptr<base::TaskRunner> GetSlowTaskRunner() const;
+
  protected:
   // Broadcasts notifications to all registered observers.
   void NotifyObserversCertDBChanged();
@@ -275,6 +285,9 @@ class NET_EXPORT NSSCertDatabase {
   // A helper observer that forwards events from this database to CertDatabase.
   std::unique_ptr<Observer> cert_notification_forwarder_;
 
+  // Task runner that should be used in tests if set.
+  scoped_refptr<base::TaskRunner> slow_task_runner_for_test_;
+
   const scoped_refptr<base::ObserverListThreadSafe<Observer>> observer_list_;
 
   base::WeakPtrFactory<NSSCertDatabase> weak_factory_;