[Sync] Handle invalid specifics field numbers gracefully

sync/engine/store_timestamps_command.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/engine/store_timestamps_command.h"
 
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/sessions/status_controller.h"
 #include "sync/sessions/sync_session.h"
 #include "sync/syncable/directory.h"
@@ skipping 10 matching lines @@
   const sync_pb::GetUpdatesResponse& updates =
       session->status_controller().updates_response().get_updates();
 
   sessions::StatusController* status = session->mutable_status_controller();
 
   // Update the progress marker tokens from the server result.  If a marker
   // was omitted for any one type, that indicates no change from the previous
   // state.
   ModelTypeSet forward_progress_types;
   for (int i = 0; i < updates.new_progress_marker_size(); ++i) {
-    ModelType model =
-        GetModelTypeFromSpecificsFieldNumber(
-            updates.new_progress_marker(i).data_type_id());
-    if (model == UNSPECIFIED || model == TOP_LEVEL_FOLDER) {
-      NOTREACHED() << "Unintelligible server response.";
+    int field_number = updates.new_progress_marker(i).data_type_id();
+    ModelType model_type = GetModelTypeFromSpecificsFieldNumber(field_number);
+    if (!IsRealDataType(model_type)) {
+      NOTREACHED() << "Unknown field number " << field_number;
       continue;
     }
-    forward_progress_types.Put(model);
-    dir->SetDownloadProgress(model, updates.new_progress_marker(i));
+    forward_progress_types.Put(model_type);
+    dir->SetDownloadProgress(model_type, updates.new_progress_marker(i));
   }
   DCHECK(!forward_progress_types.Empty() ||
          updates.changes_remaining() == 0);
   if (VLOG_IS_ON(1)) {
     DVLOG_IF(1, !forward_progress_types.Empty())
         << "Get Updates got new progress marker for types: "
         << ModelTypeSetToString(forward_progress_types)
         << " out of possible: "
         << ModelTypeSetToString(status->updates_request_types());
   }
   if (updates.has_changes_remaining()) {
     int64 changes_left = updates.changes_remaining();
     DVLOG(1) << "Changes remaining: " << changes_left;
     status->set_num_server_changes_remaining(changes_left);
   }
 
   return SYNCER_OK;
 }
 
 }  // namespace syncer