Add AttachmentMetadata to Sync's EntryKernel.

sync/protocol/attachments.proto

Index: sync/protocol/attachments.proto
diff --git a/sync/protocol/attachments.proto b/sync/protocol/attachments.proto
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index 0000000000000000000000000000000000000000..562833b1d17911a240cecd9862b9e4c23ee7f9c4
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+++ b/sync/protocol/attachments.proto
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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.
+//
+// Sync protocol for attachments.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+option retain_unknown_fields = true;
+
+package sync_pb;
+
+// A collection of attachment metadata. This proto is part of EntryKernel's "on
+// disk" representation. Private to sync.
+message AttachmentMetadata {
+}