Implement a bag of chips for sync.

sync/protocol/sync.proto

 // 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.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 445 matching lines @@
 
   // Whether to create the mobile bookmarks folder if it's not
   // already created.  Should be set to true only by mobile clients.
   optional bool create_mobile_bookmarks_folder = 1000 [default = false];
 };
 
 message AuthenticateMessage {
   required string auth_token = 1;
 };
 
+// Opaque data used by the server to track the client with.
+message ChipBag {
+};
+
 message ClientToServerMessage {
   required string share = 1;
   optional int32 protocol_version = 2 [default = 31];
   enum Contents {
     COMMIT = 1;
     GET_UPDATES = 2;
     AUTHENTICATE = 3;
     CLEAR_DATA = 4;
   }
 
   required Contents message_contents = 3;
   optional CommitMessage commit = 4;
   optional GetUpdatesMessage get_updates = 5;
   optional AuthenticateMessage authenticate = 6;
 
   // Request to clear all Chromium data from the server.
   // DEPRECATED - this field was never used in production.
   // optional ClearUserDataMessage clear_user_data = 9;
 
   optional string store_birthday = 7; // Opaque store ID; if it changes, duck!
   // The client sets this if it detects a sync issue. The server will tell it
   // if it should perform a refresh.
   optional bool sync_problem_detected = 8 [default = false];
 
   // Client side state information for debugging purpose.
   // This is only sent on the first getupdates of every sync cycle,
   // as an optimization to save bandwidth.
   optional DebugInfo debug_info = 10;
+
+  // Per-client state for use by the server. Sent with every message sent to the
+  // server.
+  optional ChipBag bag_of_chips = 11;
 };
 
 message CommitResponse {
   enum ResponseType {
     SUCCESS = 1;
     CONFLICT = 2; // You're out of date; update and check your data
     // TODO(ncarter): What's the difference between RETRY and TRANSIENT_ERROR?
     RETRY = 3; // Someone has a conflicting, non-expired session open
     INVALID_MESSAGE = 4; // What the client sent was invalid, and trying again
                          // won't help.
@@ skipping 167 matching lines @@
     optional SyncEnums.ErrorType error_type = 1 [default = UNKNOWN];
     optional string error_description       = 2;
     optional string url                     = 3;
     optional SyncEnums.Action action        = 4 [default = UNKNOWN_ACTION];
 
     // Currently only meaningful if |error_type| is throttled. If this field
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
+
+  // The new per-client state for this client. If set, should be persisted and
+  // sent with any subsequent ClientToServerMessages
+  optional ChipBag new_bag_of_chips = 14;
 };