sync: Implement per-type commit interface

sync/engine/syncer.h

Index: sync/engine/syncer.h
diff --git a/sync/engine/syncer.h b/sync/engine/syncer.h
index 132f6ef7b0cb0605458424f8d7773b0da0ed8fc2..d943e5f1206dbd4855988f4467a27936dd6f4c59 100644
--- a/sync/engine/syncer.h
+++ b/sync/engine/syncer.h
@@ -73,6 +73,14 @@ class SYNC_EXPORT_PRIVATE Syncer {
       sessions::SyncSession* session,
       base::Callback<void(sync_pb::ClientToServerMessage*)> build_fn);
 
+  // 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,
+      sessions::SyncSession* session);
+
   void HandleCycleBegin(sessions::SyncSession* session);
   bool HandleCycleEnd(
       sessions::SyncSession* session,