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
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+++ b/sync/engine/commit_contributor.h
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+// 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>
+
+#include "base/memory/scoped_ptr.h"
+
+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;
+
+  // Gathers up to |max_entries| unsynced items from this contributor into a
+  // CommitContribution.  Returns NULL when the contributor has nothing to
+  // contribute.
+  virtual scoped_ptr<CommitContribution> GetContribution(
+      size_t max_entries) = 0;
+};
+
+}  // namespace
+
+#endif  // SYNC_ENGINE_COMMIT_CONTRIBUTOR_H_