Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/history_delete_directive_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 history delete directives.
 
 // 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;
 
 // All timestamps below are from Sane Time (
 // http://www.chromium.org/developers/design-documents/sane-time )
 // and are in microseconds since the Unix epoch.
 
 // Properties of history delete directive sync objects.
 message HistoryDeleteDirectiveSpecifics {
   // Exactly one of the fields below must be filled in.  Otherwise, this
@@ skipping 15 matching lines @@
 
 message TimeRangeDirective {
   // Both fields below must be filled in.  Otherwise, this delete directive
   // must be ignored.
 
   // The time on or after which entries must be deleted.
   optional int64 start_time_usec = 1;
   // The time on or before which entries must be deleted.
   optional int64 end_time_usec = 2;
 }