[SyncFS] Use variadic template in callback_helper.h

chrome/browser/sync_file_system/drive_backend/callback_helper.h.pump

-$$ This is a pump file for generating file templates.  Pump is a python
-$$ script that is part of the Google Test suite of utilities.  Description
-$$ can be found here:
-$$
-$$ http://code.google.com/p/googletest/wiki/PumpManual
-$$
-$$ See comment for MAX_ARITY in base/bind.h.pump.
-$var MAX_ARITY = 7
-
-// Copyright 2014 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.
-
-#ifndef CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_
-#define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_
-
-#include "base/bind.h"
-#include "base/location.h"
-#include "base/logging.h"
-#include "base/sequenced_task_runner.h"
-#include "base/thread_task_runner_handle.h"
-
-// TODO(tzik): Merge this file to media/base/bind_to_current_loop.h.
-
-namespace sync_file_system {
-namespace drive_backend {
-
-namespace internal {
-
-template <typename T>
-typename base::enable_if<
-    base::internal::IsMoveOnlyType<T>::value,
-    base::internal::PassedWrapper<T> >::type
-RebindForward(T& t) {
-  return base::Passed(&t);
-}
-
-template <typename T>
-typename base::enable_if<
-    !base::internal::IsMoveOnlyType<T>::value,
-    T&>::type
-RebindForward(T& t) {
-  return t;
-}
-
-template <typename T>
-class CallbackHolder {
- public:
-  CallbackHolder(base::SequencedTaskRunner* task_runner,
-                 const tracked_objects::Location& from_here,
-                 const base::Callback<T>& callback)
-      : task_runner_(task_runner),
-        from_here_(from_here),
-        callback_(new base::Callback<T>(callback)) {
-    DCHECK(task_runner_);
-  }
-
-  ~CallbackHolder() {
-    base::Callback<T>* callback = callback_.release();
-    if (!task_runner_->DeleteSoon(from_here_, callback))
-      delete callback;
-  }
-
-  base::SequencedTaskRunner* task_runner() const { return task_runner_; }
-  const tracked_objects::Location& from_here() const { return from_here_; }
-  const base::Callback<T>& callback() const { return *callback_; }
-
- private:
-  scoped_refptr<base::SequencedTaskRunner> task_runner_;
-  const tracked_objects::Location from_here_;
-  scoped_ptr<base::Callback<T> > callback_;
-
-  DISALLOW_COPY_AND_ASSIGN(CallbackHolder);
-};
-
-template <typename>
-struct RelayToTaskRunnerHelper;
-
-$range ARITY 0..MAX_ARITY
-$for ARITY [[
-$range ARG 1..ARITY
-
-template <$for ARG , [[typename A$(ARG)]]>
-struct RelayToTaskRunnerHelper<void($for ARG , [[A$(ARG)]])> {
-  static void Run(CallbackHolder<void($for ARG , [[A$(ARG)]])>* holder
-$if ARITY != 0 [[, ]]
-$for ARG , [[A$(ARG) a$(ARG)]]
-) {
-    holder->task_runner()->PostTask(
-        holder->from_here(), base::Bind(holder->callback()
-$if ARITY != 0 [[, ]]
-$for ARG , [[RebindForward(a$(ARG))]]));
-  }
-};
-
-]]  $$ for ARITY
-
-}  // namespace internal
-
-template <typename T>
-base::Callback<T> RelayCallbackToTaskRunner(
-    base::SequencedTaskRunner* task_runner,
-    const tracked_objects::Location& from_here,
-    const base::Callback<T>& callback) {
-  DCHECK(task_runner->RunsTasksOnCurrentThread());
-
-  if (callback.is_null())
-    return base::Callback<T>();
-
-  return base::Bind(&internal::RelayToTaskRunnerHelper<T>::Run,
-                    base::Owned(new internal::CallbackHolder<T>(
-                        task_runner, from_here, callback)));
-}
-
-template <typename T>
-base::Callback<T> RelayCallbackToCurrentThread(
-    const tracked_objects::Location& from_here,
-    const base::Callback<T>& callback) {
-  return RelayCallbackToTaskRunner(
-      base::ThreadTaskRunnerHandle::Get(),
-      from_here, callback);
-}
-
-}  // namespace drive_backend
-}  // namespace sync_file_system
-
-#endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_