Extended the ProtoDatabase to provide LoadKeys() functionality.

components/leveldb_proto/proto_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_PROTO_DATABASE_H_
 #define COMPONENTS_LEVELDB_PROTO_PROTO_DATABASE_H_
 
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/callback_forward.h"
 
 namespace base {
 class FilePath;
 }
 
 namespace leveldb_proto {
 
 // Interface for classes providing persistent storage of Protocol Buffer
 // entries (T must be a Proto type extending MessageLite).
 template <typename T>
 class ProtoDatabase {
  public:
   using InitCallback = base::Callback<void(bool success)>;
   using UpdateCallback = base::Callback<void(bool success)>;
   using LoadCallback =
       base::Callback<void(bool success, std::unique_ptr<std::vector<T>>)>;
+  using LoadKeysCallback =
+      base::Callback<void(bool success,
+                          std::unique_ptr<std::vector<std::string>>)>;
   using GetCallback = base::Callback<void(bool success, std::unique_ptr<T>)>;
   using DestroyCallback = base::Callback<void(bool success)>;
 
   // A list of key-value (string, T) tuples.
   using KeyEntryVector = std::vector<std::pair<std::string, T>>;
 
   virtual ~ProtoDatabase() {}
 
   // Asynchronously initializes the object. |callback| will be invoked on the
   // calling thread when complete.
   virtual void Init(const char* client_name,
                     const base::FilePath& database_dir,
                     const InitCallback& callback) = 0;
 
   // Asynchronously saves |entries_to_save| and deletes entries from
   // |keys_to_remove| from the database. |callback| will be invoked on the
   // calling thread when complete.
   virtual void UpdateEntries(
       std::unique_ptr<KeyEntryVector> entries_to_save,
       std::unique_ptr<std::vector<std::string>> keys_to_remove,
       const UpdateCallback& callback) = 0;
 
   // Asynchronously loads all entries from the database and invokes |callback|
   // when complete.
   virtual void LoadEntries(const LoadCallback& callback) = 0;
 
+  // Asynchronously loads all keys from the database and invokes |callback| with
+  // those keys when complete.
+  virtual void LoadKeys(const LoadKeysCallback& callback) = 0;
+
   // Asynchronously loads a single entry, identified by |key|, from the database
   // and invokes |callback| when complete. If no entry with |key| is found,
   // a nullptr is passed to the callback, but the success flag is still true.
   virtual void GetEntry(const std::string& key,
                         const GetCallback& callback) = 0;
 
   // Asynchronously destroys the database.
   virtual void Destroy(const DestroyCallback& callback) = 0;
 };
 
 }  // namespace leveldb_proto
 
 #endif  // COMPONENTS_LEVELDB_PROTO_PROTO_DATABASE_H_