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

chrome/browser/sync/engine/process_updates_command.cc

Index: chrome/browser/sync/engine/process_updates_command.cc
diff --git a/chrome/browser/sync/engine/process_updates_command.cc b/chrome/browser/sync/engine/process_updates_command.cc
index e6763bdc773eeebb88ebcc0b25bd15088a05d1b0..8ee7d7b41ecbc6caafb2edf7efcc317b510428ac 100644
--- a/chrome/browser/sync/engine/process_updates_command.cc
+++ b/chrome/browser/sync/engine/process_updates_command.cc
@@ -22,10 +22,21 @@ namespace browser_sync {
 
 using sessions::SyncSession;
 using sessions::StatusController;
+using sessions::UpdateProgress;
 
 ProcessUpdatesCommand::ProcessUpdatesCommand() {}
 ProcessUpdatesCommand::~ProcessUpdatesCommand() {}
 
+bool ProcessUpdatesCommand::HasCustomGroupsToChange() const {
+  // TODO(akalin): Set to true.
+  return false;
+}
+
+std::set<ModelSafeGroup> ProcessUpdatesCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  return session.GetEnabledGroupsWithVerifiedUpdates();
+}
+
 bool ProcessUpdatesCommand::ModelNeutralExecuteImpl(SyncSession* session) {
   const GetUpdatesResponse& updates =
       session->status_controller().updates_response().get_updates();