sync: Add interfaces for per-type sync

sync/engine/commit_contributor.h

Index: sync/engine/commit_contributor.h
diff --git a/sync/engine/commit_contributor.h b/sync/engine/commit_contributor.h
new file mode 100644
index 0000000000000000000000000000000000000000..926468360c3425e354495d34fadef3389a95f4da
--- /dev/null
+++ b/sync/engine/commit_contributor.h
@@ -0,0 +1,32 @@
+// 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 SYNC_ENGINE_COMMIT_CONTRIBUTOR_H_
+#define SYNC_ENGINE_COMMIT_CONTRIBUTOR_H_
+
+#include <cstddef>
+
+namespace syncer {
+
+class CommitContribution;
+
+namespace syncable {
+class Directory;
+}
+
+// This class represents a source of items to commit to the sync server.
+//
+// When asked, it can return CommitContribution objects that contain a set of
+// items to be committed from this source.
+class CommitContributor {
+ public:
+  CommitContributor();
+  virtual ~CommitContributor() = 0;
+
+  virtual CommitContribution* GetContribution(size_t max_entries) = 0;

[tim (not reviewing), 2014/02/18 19:15:01]

Even though these are ultimately going into an STL map, return a scoped_ptr vs
raw pointer here in the interface to make ownership responsibility clear.

Also, explain the |max_entries| parameter with a comment.

[rlarocque, 2014/02/18 20:01:13]

Done.

+};
+
+}  // namespace
+
+#endif  // SYNC_ENGINE_COMMIT_CONTRIBUTOR_H_