Ensure tests have an active task runner

net/disk_cache/simple/simple_backend_impl.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/disk_cache/simple/simple_backend_impl.h"
 
 #include <algorithm>
 #include <cstdlib>
 #include <functional>
 
@@ skipping 717 matching lines @@
     int result) {
   std::for_each(
       entry_hashes->begin(), entry_hashes->end(),
       std::bind1st(std::mem_fun(&SimpleBackendImpl::OnDoomComplete),
                    this));
   callback.Run(result);
 }
 
 // static
 void SimpleBackendImpl::FlushWorkerPoolForTesting() {
-  g_sequenced_worker_pool.Get().FlushForTesting();
+  // We only need to do this if we there is an active task runner.
+  if (base::ThreadTaskRunnerHandle::IsSet())
+    g_sequenced_worker_pool.Get().FlushForTesting();
 }
 
 }  // namespace disk_cache