[SyncFS] Introduce CallbackTracker to handle callback abortion

chrome/browser/sync_file_system/drive_backend/callback_tracker_internal.cc

+// 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.
+
+#include "chrome/browser/sync_file_system/drive_backend/callback_tracker_internal.h"
+
+#include "chrome/browser/sync_file_system/drive_backend/callback_tracker.h"
+
+namespace sync_file_system {
+namespace drive_backend {
+namespace internal {
+
+AbortHelper::AbortHelper(CallbackTracker* tracker)
+    : tracker_(tracker), weak_ptr_factory_(this) {
+}
+
+AbortHelper::~AbortHelper() {}
+
+base::WeakPtr<AbortHelper> AbortHelper::AsWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
+// static
+scoped_ptr<AbortHelper> AbortHelper::TakeOwnership(
+    const base::WeakPtr<AbortHelper>& abort_helper) {
+  if (!abort_helper)
+    return scoped_ptr<AbortHelper>();
+  scoped_ptr<AbortHelper> result =
+      abort_helper->tracker_->PassAbortHelper(abort_helper.get());
+  abort_helper->weak_ptr_factory_.InvalidateWeakPtrs();
+  return result.Pass();
+}
+
+}  // namespace internal
+}  // namespace drive_backend
+}  // namespace sync_file_system