sync: Add interfaces for per-type sync

sync/engine/directory_commit_contributor.cc

+// Copyright 2013 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.
+
+#include "sync/engine/directory_commit_contributor.h"
+
+#include "sync/engine/directory_commit_contribution.h"
+
+namespace syncer {
+
+DirectoryCommitContributor::DirectoryCommitContributor(
+    syncable::Directory* dir,
+    ModelType type)
+    : dir_(dir),
+      type_(type) {}
+
+DirectoryCommitContributor::~DirectoryCommitContributor() {}
+
+scoped_ptr<CommitContribution>
+DirectoryCommitContributor::GetContribution(size_t max_entries) {
+  return DirectoryCommitContribution::Build(dir_, type_, max_entries)
+      .PassAs<CommitContribution>();
+}
+
+}  // namespace syncer