Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/sync_enums.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 SyncEnums {
   // These events are sent by the DebugInfo class for singleton events.
   enum SingletonDebugEventType {
     CONNECTION_STATUS_CHANGE = 1;  // Connection status change. Note this
                                    // gets generated even during a successful
                                    // connection.
     UPDATED_TOKEN = 2;             // Client received an updated token.
@@ skipping 134 matching lines @@
                            // user opted to sync a different set of datatypes.
     GU_TRIGGER = 12;  // The client is in 'normal' mode.  It may have several
                       // reasons for requesting an update.  See the per-type
                       // GetUpdateTriggers message for more details.
     RETRY = 13;       // A retry GU to pick up updates missed by last GU due to
                       // replication delay, missing hints, etc.
     PROGRAMMATIC = 14;  // A GU to programmatically enable/disable a
                         // datatype, often due to error handling.
   }
 }