Cleanup: remove non-standard retain_unknown_fields from .proto files

components/sync/protocol/session_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 sessions.
 
 // 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;
 
 import "sync_enums.proto";
 
 message SessionSpecifics {
   // Unique id for the client.
   optional string session_tag = 1;
   optional SessionHeader header = 2;
   optional SessionTab tab = 3;
@@ skipping 132 matching lines @@
   }
   // Whether the Password Manager saw a password field on the page.
   optional PasswordState password_state = 26;
 }
 
 // Navigation information for a single redirection within a single navigation.
 message NavigationRedirect {
   // A URL that redirected while navigating to the virtual_url.
   optional string url = 1;
 }