[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/update_applicator.cc

 // Copyright (c) 2011 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 "chrome/browser/sync/engine/update_applicator.h"
 
 #include <vector>
 
 #include "base/logging.h"
 #include "chrome/browser/sync/engine/syncer_util.h"
@@ skipping 79 matching lines @@
 }
 
 void UpdateApplicator::Advance() {
   --end_;
   *pointer_ = *end_;
 }
 
 bool UpdateApplicator::SkipUpdate(const syncable::Entry& entry) {
   syncable::ModelType type = entry.GetServerModelType();
   ModelSafeGroup g = GetGroupForModelType(type, routing_info_);
-  // The extra routing_info count check here is to support GetUpdateses for
-  // a subset of the globally enabled types, and not attempt to update items
-  // if their type isn't permitted in the current run.  These would typically
-  // be unapplied items from a previous sync.
-  if (g != group_filter_)
+  // The set of updates passed to the UpdateApplicator should already
+  // be group-filtered.
+  if (g != group_filter_) {
+    NOTREACHED();
     return true;
+  }
   if (g == GROUP_PASSIVE &&
       !routing_info_.count(type) &&
       type != syncable::UNSPECIFIED &&
       type != syncable::TOP_LEVEL_FOLDER) {
     DVLOG(1) << "Skipping update application, type not permitted.";
     return true;
   }
   return false;
 }
 
@@ skipping 54 matching lines @@
 void UpdateApplicator::ResultTracker::ClearConflicts() {
   conflicting_ids_.clear();
   encryption_conflict_ids_.clear();
 }
 
 bool UpdateApplicator::ResultTracker::no_conflicts() const {
   return conflicting_ids_.empty();
 }
 
 }  // namespace browser_sync