Ensure tests have an active task runner

net/disk_cache/simple/simple_backend_impl.cc

Index: net/disk_cache/simple/simple_backend_impl.cc
diff --git a/net/disk_cache/simple/simple_backend_impl.cc b/net/disk_cache/simple/simple_backend_impl.cc
index 38086441ebda528f93034956d4e85eb94945cbd3..b5a6c9ef36221bc20dc8faf9e4af7c0be6ca172a 100644
--- a/net/disk_cache/simple/simple_backend_impl.cc
+++ b/net/disk_cache/simple/simple_backend_impl.cc
@@ -735,7 +735,9 @@ void SimpleBackendImpl::DoomEntriesComplete(
 
 // 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