Remove dependency on BrowserThread::FILE from Sync code

chrome/browser/extensions/api/storage/backend_task_runner.h

Index: chrome/browser/extensions/api/storage/backend_task_runner.h
diff --git a/chrome/browser/extensions/api/storage/backend_task_runner.h b/chrome/browser/extensions/api/storage/backend_task_runner.h
new file mode 100644
index 0000000000000000000000000000000000000000..5bdb521b0d3a753157969a18cd066e437d82d141
--- /dev/null
+++ b/chrome/browser/extensions/api/storage/backend_task_runner.h
@@ -0,0 +1,18 @@
+// Copyright (c) 2017 The Chromium Authors. All rights reserved.

[Devlin, 2017/06/21 00:25:21]

no (c) in new files

[stanisc, 2017/06/21 17:20:54]

Done.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_
+#define CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_
+
+#include "base/memory/ref_counted.h"
+#include "base/sequenced_task_runner.h"
+
+namespace extensions {
+
+// Gets the singleton task runner for running Extensions storage backend tasks.

[Devlin, 2017/06/21 00:25:20]

Maybe explain what a "storage backend" task includes?

[stanisc, 2017/06/21 17:20:55]

I don't actually know exactly what needs to be posted on this task runner.
I think at a minimum anything related to SyncStorageBackend needs to be on this
runner to ensure that the same sequence is shared with the sync.
It looks like both StorageFrontend and SyncValueStoreCache need to be switched
but likely more extensions code that is currently posting on the FILE thread
might need to be switched to this task runner as well. I'd appreciate your help
in figuring that out. I'll follow up on that in a separate patch.

[Devlin, 2017/06/21 19:19:30]

Yeah, I'm sure there will be more code to be updated, but having this CL
targeted is good.  I like your new phrasing since it's clear what this is used
for currently.

+scoped_refptr<base::SequencedTaskRunner> GetBackendTaskRunner();
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_