Extended the ProtoDatabase to provide LoadKeys() functionality.

components/leveldb_proto/leveldb_database.h

Index: components/leveldb_proto/leveldb_database.h
diff --git a/components/leveldb_proto/leveldb_database.h b/components/leveldb_proto/leveldb_database.h
index 4474a87ca644160a20e404eb200cf18900aaa25c..8c984b797fc717ad96573b8d98909dbcf2b38a43 100644
--- a/components/leveldb_proto/leveldb_database.h
+++ b/components/leveldb_proto/leveldb_database.h
@@ -45,6 +45,7 @@ class LevelDB : base::trace_event::MemoryDumpProvider {
   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);