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

chrome/browser/sync/engine/update_applicator.cc

Index: chrome/browser/sync/engine/update_applicator.cc
diff --git a/chrome/browser/sync/engine/update_applicator.cc b/chrome/browser/sync/engine/update_applicator.cc
index 6246836e0189008962bcf9c1f0b3ab2b519e39af..e44083cf69e1a0e3d8b6cfa7584a3948ad6b920a 100644
--- a/chrome/browser/sync/engine/update_applicator.cc
+++ b/chrome/browser/sync/engine/update_applicator.cc
@@ -97,12 +97,12 @@ void UpdateApplicator::Advance() {
 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 &&