sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/protocol/autofill_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 autofill.
 
 // 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.proto";
-
 // Properties of autofill sync objects.
 
 // An AutofillProfile.
 message AutofillProfileSpecifics {
   // User-defined label.
   optional string label = 1;
 
   optional string guid = 15;
 
   // Contact info.
@@ skipping 29 matching lines @@
   // An autofill++ profile object.  If present, indicates this entity
   // represents an AutofillProfile exclusively, and no other fields (such as
   // name/value or credit_card) should be present.
   optional AutofillProfileSpecifics profile = 4;
 
   // Obsolete credit card fields.
   // optional bytes deprecated_encrypted_credit_card = 5;
   // optional AutofillCreditCardSpecifics deprecated_credit_card = 6;
 }
 
-extend EntitySpecifics {
-  optional AutofillSpecifics autofill = 31729;
-  optional AutofillProfileSpecifics autofill_profile = 63951;
-}