Add attachments support to DomDistillerStore

sync/protocol/article_specifics.proto

 // Copyright 2013 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 datatype extension for the article.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
+import "attachments.proto";
+
 // Properties of Article objects.
 message ArticleSpecifics {
   // Next ID to use: 4
 
   optional string entry_id = 1;
 
   optional string title = 2;
 
   repeated ArticlePage pages = 3;
+
+  optional ArticleAttachments attachments = 4;
 }
 
 message ArticlePage {
   // Next ID to use: 2
 
   optional string url = 1;
 }
+
+message ArticleAttachments {
+  optional AttachmentIdProto distilled_article = 1;
+}