Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/experiments_specifics.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 datatype extension for experimental feature flags.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
-option retain_unknown_fields = true;
 
 package sync_pb;
 
 // A flag to enable support for keystore encryption.
 message KeystoreEncryptionFlags {
   optional bool enabled = 1;
 }
 
 // Whether history delete directives are enabled.
 message HistoryDeleteDirectives {
@@ skipping 48 matching lines @@
   optional AutofillCullingFlags autofill_culling = 3;
   optional FaviconSyncFlags favicon_sync = 4;
   optional PreCommitUpdateAvoidanceFlags pre_commit_update_avoidance = 5;
   optional GcmChannelFlags gcm_channel = 6;
   // No longer used as of M43.
   optional EnhancedBookmarksFlags obsolete_enhanced_bookmarks = 7;
   optional GcmInvalidationsFlags gcm_invalidations = 8;
   // No longer used as of M51.
   optional WalletSyncFlags obsolete_wallet_sync = 9;
 }