Make the throttle delay configurable by the server.

chrome/browser/sync/protocol/sync.proto

 // Copyright (c) 2011 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 568 matching lines @@
   // Time to wait before sending any requests to the server.
   optional int32 set_sync_poll_interval = 1;  // in seconds
   optional int32 set_sync_long_poll_interval = 2;  // in seconds
 
   optional int32 max_commit_batch_size = 3;
 
   // Number of seconds to delay between a sessions
   // action and sending a commit message to the
   // server
   optional int32 sessions_commit_delay_seconds = 4;
+
+  // Number of seconds to delay before the throttled client should retry.
+  optional int32 throttle_delay_seconds = 5;
 };
 
 message ClientToServerResponse {
   optional CommitResponse commit = 1;
   optional GetUpdatesResponse get_updates = 2;
   optional AuthenticateResponse authenticate = 3;
   optional ClearUserDataResponse clear_user_data = 9;
   optional GetUpdatesMetadataResponse stream_metadata = 10;
   // If GetUpdatesStreamingResponse is contained in the ClientToServerResponse,
   // none of the other fields (error_code and etc) will be set.
@@ skipping 42 matching lines @@
   optional string store_birthday = 6;
 
   optional ClientCommand client_command = 7;
   optional ProfilingData profiling_data = 8;
 
   // The data types whose storage has been migrated.  Present when the value of
   // error_code is MIGRATION_DONE.
   repeated int32 migrated_data_type_id = 12;
 };