sync: Expose sync functionality as functions

sync/engine/get_commit_ids_command.h

Index: sync/engine/get_commit_ids_command.h
diff --git a/sync/engine/get_commit_ids_command.h b/sync/engine/get_commit_ids_command.h
index 02d6766585eea7d729eaf97b523586f0e28e86eb..4145d606059602f95ac0438749e5ab46c8894a8f 100644
--- a/sync/engine/get_commit_ids_command.h
+++ b/sync/engine/get_commit_ids_command.h
@@ -40,6 +40,7 @@ class SYNC_EXPORT_PRIVATE GetCommitIdsCommand : public SyncerCommand {
   // provided batch_size.  This contents of this "set" will be ordered; see the
   // comments in this class' implementation for details.
   GetCommitIdsCommand(syncable::BaseTransaction* trans,
+                      ModelTypeSet requested_types,
                       const size_t commit_batch_size,
                       sessions::OrderedCommitSet* ordered_commit_set);
   virtual ~GetCommitIdsCommand();
@@ -122,6 +123,9 @@ class SYNC_EXPORT_PRIVATE GetCommitIdsCommand : public SyncerCommand {
   // A pointer to a valid transaction not owned by this class.
   syncable::BaseTransaction* trans_;
 
+  // The set of types from which to draw commit IDs.
+  const ModelTypeSet requested_types_;
+
   // Input parameter; see constructor comment.
   const size_t requested_commit_batch_size_;