Implement ProtoDatabase::GetEntry

components/leveldb_proto/proto_database.h

Index: components/leveldb_proto/proto_database.h
diff --git a/components/leveldb_proto/proto_database.h b/components/leveldb_proto/proto_database.h
index 30949bf3bdf3ddd25b4c74a5a311507dde2abcd4..20c630e1ee9ebe8967bfd418171736f93fd0111f 100644
--- a/components/leveldb_proto/proto_database.h
+++ b/components/leveldb_proto/proto_database.h
@@ -27,6 +27,7 @@ class ProtoDatabase {
   using UpdateCallback = base::Callback<void(bool success)>;
   using LoadCallback =
       base::Callback<void(bool success, std::unique_ptr<std::vector<T>>)>;
+  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.
@@ -52,6 +53,12 @@ class ProtoDatabase {
   // when complete.
   virtual void LoadEntries(const LoadCallback& 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;
 };