Implement a bag of chips for sync.

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index b34882feb40ee319598b8832109ccc05e4704478..49d598cc98ef843edf19a0c0b0194a96f8c5ed17 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -170,6 +170,10 @@ class Directory {
     int64 next_id;
     // The persisted notification state.
     std::string notification_state;
+    // The serialized bag of chips we were given by the server. Contents are
+    // opaque to the client. This is the serialization of a message of type
+    // ChipBag defined in sync.proto. It can contains NULL characters.
+    std::string bag_of_chips;
   };
 
   // What the Directory needs on initialization to create itself and its Kernel.
@@ -260,6 +264,11 @@ class Directory {
   std::string store_birthday() const;
   void set_store_birthday(const std::string& store_birthday);
 
+  // (Account) Bag of chip is an opaque state used by the server to track the
+  // client.
+  std::string bag_of_chips() const;
+  void set_bag_of_chips(const std::string& bag_of_chips);
+
   std::string GetNotificationState() const;
   void SetNotificationState(const std::string& notification_state);