[Sync] Handle invalid specifics field numbers gracefully

sync/syncable/model_type.cc

 // 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.
 
 #include "sync/internal_api/public/base/model_type.h"
 
 #include "base/string_split.h"
 #include "base/values.h"
 #include "sync/protocol/app_notification_specifics.pb.h"
 #include "sync/protocol/app_setting_specifics.pb.h"
@@ skipping 75 matching lines @@
       NOTREACHED() << "No known extension for model type.";
   }
 }
 
 ModelType GetModelTypeFromSpecificsFieldNumber(int field_number) {
   for (int i = FIRST_REAL_MODEL_TYPE; i < MODEL_TYPE_COUNT; ++i) {
     ModelType model_type = ModelTypeFromInt(i);
     if (GetSpecificsFieldNumberFromModelType(model_type) == field_number)
       return model_type;
   }
-  NOTREACHED();
   return UNSPECIFIED;
 }
 
 int GetSpecificsFieldNumberFromModelType(ModelType model_type) {
   switch (model_type) {
     case BOOKMARKS:
       return sync_pb::EntitySpecifics::kBookmarkFieldNumber;
       break;
     case PASSWORDS:
       return sync_pb::EntitySpecifics::kPasswordFieldNumber;
@@ skipping 520 matching lines @@
   }
   *model_type = UNSPECIFIED;
   return false;
 }
 
 bool IsRealDataType(ModelType model_type) {
   return model_type >= FIRST_REAL_MODEL_TYPE && model_type < MODEL_TYPE_COUNT;
 }
 
 }  // namespace syncer