sync: Implement per-type commit interface

sync/engine/sync_directory_commit_contributor.h

Index: sync/engine/sync_directory_commit_contributor.h
diff --git a/sync/engine/sync_directory_commit_contributor.h b/sync/engine/sync_directory_commit_contributor.h
new file mode 100644
index 0000000000000000000000000000000000000000..c7c03e5d27757e8efa31d022632be266ab1cfdba
--- /dev/null
+++ b/sync/engine/sync_directory_commit_contributor.h
@@ -0,0 +1,45 @@
+// Copyright (c) 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.
+
+#ifndef SYNC_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_
+#define SYNC_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_
+
+#include <map>
+
+#include "sync/internal_api/public/base/model_type.h"
+
+namespace syncer {
+
+class SyncDirectoryCommitContribution;
+
+namespace syncable {
+class Directory;
+}
+
+// This class represents the syncable::Directory as a source of items to commit
+// to the sync server.
+//
+// Each instance of this class represents a particular type within the
+// syncable::Directory.  When asked, it will iterate through the directory, grab
+// any items of its type that are ready for commit, and return them in the form
+// of a SyncDirectoryCommitContribution.
+class SyncDirectoryCommitContributor {
+ public:
+  SyncDirectoryCommitContributor(syncable::Directory* dir, ModelType type);
+  ~SyncDirectoryCommitContributor();
+
+  SyncDirectoryCommitContribution* GetContribution(size_t max_entries);
+
+ private:
+  syncable::Directory* dir_;
+  ModelType type_;
+};
+
+// TODO(rlarocque): Find a better place for this definition.
+typedef std::map<ModelType, SyncDirectoryCommitContributor*>
+    CommitContributorMap;
+
+}  // namespace
+
+#endif  // SYNC_ENGINE_SYNC_DIRECTORY_COMMIT_CONTRIBUTOR_H_