sync: Implement per-type update processing

sync/engine/download.h

Index: sync/engine/download.h
diff --git a/sync/engine/download.h b/sync/engine/download.h
index ddae79b115be315910f2426daaec408b12c1a5ba..7c5c5829a5a8bd3037cc6cc1482c79b194124814 100644
--- a/sync/engine/download.h
+++ b/sync/engine/download.h
@@ -55,7 +55,8 @@ SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForPoll(
 // Sends the specified message to the server and stores the response in a member
 // of the |session|'s StatusController.
 SYNC_EXPORT_PRIVATE SyncerError
-    ExecuteDownloadUpdates(sessions::SyncSession* session,
+    ExecuteDownloadUpdates(ModelTypeSet request_types,

[rlarocque, 2013/10/24 20:47:27]

Some of the issues I ran into in this change made me question whether or not we
should allow ProxyTypes to be sent down to this layer.  Maybe we should filter
them out at the SyncScheduler, or even earlier.

Thoughts?

+                           sessions::SyncSession* session,
                            sync_pb::ClientToServerMessage* msg);
 
 }  // namespace syncer