[Sync] Support commit only types.

components/sync/engine_impl/syncer.cc

Index: components/sync/engine_impl/syncer.cc
diff --git a/components/sync/engine_impl/syncer.cc b/components/sync/engine_impl/syncer.cc
index ccdcb70598322dbde8c4856f474d2d60989c6666..60a2194e9f39ef827eb1ac5dc8ab8bd523c12230 100644
--- a/components/sync/engine_impl/syncer.cc
+++ b/components/sync/engine_impl/syncer.cc
@@ -114,14 +114,20 @@ bool Syncer::DownloadAndApplyUpdates(ModelTypeSet* request_types,
                                      SyncCycle* cycle,
                                      const GetUpdatesDelegate& delegate,
                                      bool create_mobile_bookmarks_folder) {
+  ModelTypeSet add_for_commit = Intersection(*request_types, CommitOnlyTypes());

[Patrick Noland, 2017/06/01 22:58:48]

This name made it harder for me to follow what you're doing here. Maybe
requested_commit_only_types would be easier to follow? And a comment explaining
why you remove these types for download and then add them back for the GU
request?

[skym, 2017/06/01 23:15:21]

Done.

+  ModelTypeSet download_types = Difference(*request_types, add_for_commit);
   GetUpdatesProcessor get_updates_processor(
       cycle->context()->model_type_registry()->update_handler_map(), delegate);
   SyncerError download_result = UNSET;
   do {
     download_result = get_updates_processor.DownloadUpdates(
-        request_types, cycle, create_mobile_bookmarks_folder);
+        &download_types, cycle, create_mobile_bookmarks_folder);
   } while (download_result == SERVER_MORE_TO_DOWNLOAD);
 
+  // It is our responsibility to propagate the removal of types that occurred in
+  // GetUpdatesProcessor::DownloadUpdates().
+  *request_types = Union(download_types, add_for_commit);
+
   // Exit without applying if we're shutting down or an error was detected.
   if (download_result != SYNCER_OK || ExitRequested())
     return false;
@@ -135,7 +141,7 @@ bool Syncer::DownloadAndApplyUpdates(ModelTypeSet* request_types,
     // Apply updates to the other types. May or may not involve cross-thread
     // traffic, depending on the underlying update handlers and the GU type's
     // delegate.
-    get_updates_processor.ApplyUpdates(*request_types,
+    get_updates_processor.ApplyUpdates(download_types,
                                        cycle->mutable_status_controller());
 
     cycle->context()->set_hierarchy_conflict_detected(