Unrevert r141537: Add extensions::StateStore and use that instead of

chrome/browser/value_store/value_store_frontend.h

+// Copyright (c) 2012 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_VALUE_STORE_VALUE_STORE_FRONTEND_H_
+#define CHROME_BROWSER_VALUE_STORE_VALUE_STORE_FRONTEND_H_
+#pragma once
+
+#include <string>
+
+#include "base/callback.h"
+#include "base/file_path.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/non_thread_safe.h"
+#include "base/values.h"
+
+// A frontend for a LeveldbValueStore, for use on the UI thread.
+class ValueStoreFrontend
+    : public base::SupportsWeakPtr<ValueStoreFrontend>,
+      public base::NonThreadSafe {
+ public:
+  typedef base::Callback<void(scoped_ptr<base::Value>)> ReadCallback;
+
+  explicit ValueStoreFrontend(const FilePath& db_path);
+  ~ValueStoreFrontend();
+
+  // Retrieves a value from the database asynchronously, passing a copy to
+  // |callback| when ready. NULL is passed if no matching entry is found.
+  void Get(const std::string& key, const ReadCallback& callback);
+
+  // Sets a value with the given key.
+  void Set(const std::string& key, scoped_ptr<base::Value> value);
+
+  // Removes the value with the given key.
+  void Remove(const std::string& key);
+
+ private:
+  class Backend;
+
+  // A helper class to manage lifetime of the backing ValueStore, which lives
+  // on the FILE thread.
+  scoped_refptr<Backend> backend_;
+
+  DISALLOW_COPY_AND_ASSIGN(ValueStoreFrontend);
+};
+
+#endif  // CHROME_BROWSER_VALUE_STORE_VALUE_STORE_FRONTEND_H_