Extended the ProtoDatabase to provide LoadKeys() functionality.

components/leveldb_proto/leveldb_database.h

 // 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 COMPONENTS_LEVELDB_PROTO_LEVELDB_DATABASE_H_
 #define COMPONENTS_LEVELDB_PROTO_LEVELDB_DATABASE_H_
 
 #include <memory>
 #include <string>
 #include <vector>
@@ skipping 26 matching lines @@
   // change once shipped.
   explicit LevelDB(const char* client_name);
   virtual ~LevelDB();
 
   virtual bool InitWithOptions(const base::FilePath& database_dir,
                                const leveldb::Options& options);
   virtual bool Init(const base::FilePath& database_dir);
   virtual bool Save(const base::StringPairs& pairs_to_save,
                     const std::vector<std::string>& keys_to_remove);
   virtual bool Load(std::vector<std::string>* entries);
+  virtual bool LoadKeys(std::vector<std::string>* keys);
   virtual bool Get(const std::string& key, bool* found, std::string* entry);
 
   static bool Destroy(const base::FilePath& database_dir);
 
  private:
   DFAKE_MUTEX(thread_checker_);
 
   // The declaration order of these members matters: |db_| depends on |env_| and
   // therefore has to be destructed first.
   std::unique_ptr<leveldb::Env> env_;
   std::unique_ptr<leveldb::DB> db_;
   base::HistogramBase* open_histogram_;
 
   DISALLOW_COPY_AND_ASSIGN(LevelDB);
 };
 
 }  // namespace leveldb_proto
 
 #endif  // COMPONENTS_LEVELDB_PROTO_LEVELDB_DATABASE_H_