[Sync] Add support for server controlled nudge delays

sync/protocol/client_commands.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";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
+message CustomNudgeDelay {
+  optional int32 datatype_id = 1;  // Datatype id.
+  optional int32 delay_ms = 2;     // Delay in milliseconds.
+};
+
 message ClientCommand {
   // 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
+  // Number of seconds to delay between a sessions action and sending a commit
+  // message to the server. Can be overridden by |custom_nudge_delays|.
   optional int32 sessions_commit_delay_seconds = 4;
 
   // Number of seconds to delay before the throttled client should retry.
   optional int32 throttle_delay_seconds = 5;
 
   // Maximum number of local nudges to buffer per-type.
   optional int32 client_invalidation_hint_buffer_size = 6;
 
   // Time to wait before issuing a retry GU.
   optional int32 gu_retry_delay_seconds = 7;
+
+  // A dictionary of custom nudge delays.
+  // Note: if a SESSIONS value is present, this will override
+  // |sessions_commit_delay_seconds|
+  // New in M39.
+  repeated CustomNudgeDelay custom_nudge_delays = 8;
 };