Linux: change protobuf default option to allow building

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;
 }
 
 // Contains one flag or set of related flags.  Each node of the experiments type
 // will have a unique_client_tag identifying which flags it contains.  By
 // convention, the tag name should match the sub-message name.
 message ExperimentsSpecifics {
   optional KeystoreEncryptionFlags keystore_encryption = 1;
 }