Disk Cache: Use the old WorkerPool for Async IO on iOS.

net/disk_cache/backend_impl.cc

Index: net/disk_cache/backend_impl.cc
diff --git a/net/disk_cache/backend_impl.cc b/net/disk_cache/backend_impl.cc
index 33b48305cf5bd98df495be3b27ccdb86d7cbc91c..d10a19d9af09e76c553c99c9a54d07a0d8c30f6a 100644
--- a/net/disk_cache/backend_impl.cc
+++ b/net/disk_cache/backend_impl.cc
@@ -310,6 +310,8 @@ void BackendImpl::CleanupCache() {
       // This is a net_unittest, verify that we are not 'leaking' entries.
       File::WaitForPendingIO(&num_pending_io_);
       DCHECK(!num_refs_);
+    } else {
+      File::DropPendingIO();
     }
   }
   block_files_.CloseFiles();