[Sync] Clean up ModelType code.

components/browser_sync/profile_sync_service.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 "components/browser_sync/profile_sync_service.h"
 
 #include <stddef.h>
 
 #include <cstddef>
 #include <map>
@@ skipping 1500 matching lines @@
         syncer::user_selectable_type::TYPED_URLS,
         syncer::user_selectable_type::EXTENSIONS,
         syncer::user_selectable_type::APPS,
 #if BUILDFLAG(ENABLE_READING_LIST)
         syncer::user_selectable_type::READING_LIST,
 #endif
         syncer::user_selectable_type::PROXY_TABS,
       };
 
   static_assert(39 == syncer::MODEL_TYPE_COUNT,
-                "custom config histogram must be updated");
+                "If adding a user selectable type, update "
+                "UserSelectableSyncType in user_selectable_sync_type.h and "
+                "histograms.xml.");
 
   if (!sync_everything) {
     const syncer::ModelTypeSet current_types = GetPreferredDataTypes();
 
     syncer::ModelTypeSet type_set = syncer::UserSelectableTypes();
     syncer::ModelTypeSet::Iterator it = type_set.First();
 
     DCHECK_EQ(arraysize(user_selectable_types), type_set.Size());
 
     for (size_t i = 0; i < arraysize(user_selectable_types) && it.Good();
@@ skipping 888 matching lines @@
 
   DCHECK(startup_controller_->IsSetupInProgress());
   startup_controller_->SetSetupInProgress(false);
 
   if (IsEngineInitialized())
     ReconfigureDatatypeManager();
   NotifyObservers();
 }
 
 }  // namespace browser_sync