Add attachments support to DomDistillerStore

sync/protocol/article_specifics.proto

Index: sync/protocol/article_specifics.proto
diff --git a/sync/protocol/article_specifics.proto b/sync/protocol/article_specifics.proto
index 00631fd16ece787eb3951c02129e2c37266b05fe..436fc2e40a125f7c334cce99e7d9b7ab652831b0 100644
--- a/sync/protocol/article_specifics.proto
+++ b/sync/protocol/article_specifics.proto
@@ -11,19 +11,23 @@ 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;
+}