[Sync] Add support for performing a GetKey on startup.

sync/engine/get_key_command.h

+// Copyright (c) 2012 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_GET_KEY_COMMAND_H_
+#define SYNC_ENGINE_GET_KEY_COMMAND_H_
+
+#include "base/compiler_specific.h"
+#include "sync/engine/syncer_command.h"
+
+namespace syncer {
+
+// Sends the GetKey message to the sync server.
+class GetKeyCommand : public SyncerCommand {
+ public:
+  explicit GetKeyCommand();
+  virtual ~GetKeyCommand();
+
+  // SyncerCommand implementation.
+  virtual SyncerError ExecuteImpl(sessions::SyncSession* session) OVERRIDE;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(GetKeyCommand);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_GET_KEY_COMMAND_H_