Make it clear what last_sync_timestamp actually tracks. Update

chrome/browser/sync/engine/store_timestamps_command.h

+// Copyright (c) 2010 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 CHROME_BROWSER_SYNC_ENGINE_STORE_TIMESTAMPS_COMMAND_H_
+#define CHROME_BROWSER_SYNC_ENGINE_STORE_TIMESTAMPS_COMMAND_H_
+
+#include "chrome/browser/sync/engine/syncer_command.h"
+#include "chrome/browser/sync/engine/syncer_types.h"
+
+namespace browser_sync {
+
+// A syncer command that extracts the changelog timestamp information from
+// a GetUpdatesResponse (fetched in DownloadUpdatesCommand) and stores
+// it in the directory.  This is meant to run immediately after
+// ProcessUpdatesCommand.
+//
+// Preconditions - all updates in the SyncerSesssion have been stored in the
+//                 database, meaning it is safe to update the persisted
+//                 timestamps.
+//
+// Postconditions - The next_timestamp returned by the server will be
+//                  saved into the directory (where it will be used
+//                  the next time that DownloadUpdatesCommand runs).
+class StoreTimestampsCommand : public SyncerCommand {
+ public:
+  StoreTimestampsCommand();
+  virtual ~StoreTimestampsCommand();
+
+  // SyncerCommand implementation.
+  virtual void ExecuteImpl(sessions::SyncSession* session);
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(StoreTimestampsCommand);
+};
+
+}  // namespace browser_sync
+
+#endif  // CHROME_BROWSER_SYNC_ENGINE_STORE_TIMESTAMPS_COMMAND_H_