[Sync] Minor refactor around SyncCycle and ModelTypeSet usage.

components/sync/engine_impl/syncer.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_ENGINE_IMPL_SYNCER_H_
 #define COMPONENTS_SYNC_ENGINE_IMPL_SYNCER_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 40 matching lines @@
                                SyncCycle* cycle);
 
   // Performs an initial download for the |request_types|.  It is assumed that
   // the specified types have no local state, and that their associated change
   // processors are in "passive" mode, so none of the downloaded updates will be
   // applied to the model.  The |source| is sent up to the server for debug
   // purposes.  It describes the reson for performing this initial download.
   // Returns: false if an error occurred and retries should backoff, true
   // otherwise.
   virtual bool ConfigureSyncShare(
-      ModelTypeSet request_types,
+      const ModelTypeSet& request_types,
       sync_pb::GetUpdatesCallerInfo::GetUpdatesSource source,
       SyncCycle* cycle);
 
   // Requests to download updates for the |request_types|.  For a well-behaved
   // client with a working connection to the invalidations server, this should
   // be unnecessary.  It may be invoked periodically to try to keep the client
   // in sync despite bugs or transient failures.
   // Returns: false if an error occurred and retries should backoff, true
   // otherwise.
   virtual bool PollSyncShare(ModelTypeSet request_types, SyncCycle* cycle);
 
   // Posts a ClearServerData command.
   // Returns: false if an error occurred and retries should backoff, true
   // otherwise.
   virtual bool PostClearServerData(SyncCycle* cycle);
 
  private:
   bool DownloadAndApplyUpdates(ModelTypeSet* request_types,
                                SyncCycle* cycle,
                                const GetUpdatesDelegate& delegate,
                                bool create_mobile_bookmarks_folder);
 
   // This function will commit batches of unsynced items to the server until the
   // number of unsynced and ready to commit items reaches zero or an error is
   // encountered.  A request to exit early will be treated as an error and will
   // abort any blocking operations.
-  SyncerError BuildAndPostCommits(ModelTypeSet request_types,
+  SyncerError BuildAndPostCommits(const ModelTypeSet& request_types,
                                   NudgeTracker* nudge_tracker,
                                   SyncCycle* cycle,
                                   CommitProcessor* commit_processor);
 
   // Whether an early exist was requested due to a cancelation signal.
   bool ExitRequested();
 
   bool HandleCycleEnd(SyncCycle* cycle,
                       sync_pb::GetUpdatesCallerInfo::GetUpdatesSource source);
 
   CancelationSignal* const cancelation_signal_;
 
   // Whether the syncer is in the middle of a sync attempt.
   bool is_syncing_;
 
   DISALLOW_COPY_AND_ASSIGN(Syncer);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_IMPL_SYNCER_H_