Disk Cache: Replace the worker pool with a sequenced worker pool (posix).

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 8d7fd461102abafb528e785d5103e8f1197d026d..cb2832f1b1cc4b5c79891e36ed390fa79e0dce15 100644
--- a/net/disk_cache/backend_impl.cc
+++ b/net/disk_cache/backend_impl.cc
@@ -337,8 +337,6 @@ 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();