sync: Introduce ModelTypeRegistry and helpers

sync/engine/download.h

Index: sync/engine/download.h
diff --git a/sync/engine/download.h b/sync/engine/download.h
index 5bc08d434e1bb9a8b868cc694070a2c3f4d529a2..043fb2ffadd3988ea3a2871e3c5775c75bde5c64 100644
--- a/sync/engine/download.h
+++ b/sync/engine/download.h
@@ -6,7 +6,7 @@
 #define SYNC_ENGINE_DOWNLOAD_H_
 
 #include "sync/base/sync_export.h"
-#include "sync/engine/sync_directory_update_handler.h"
+#include "sync/engine/updater_list.h"
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/internal_api/public/util/syncer_error.h"
 #include "sync/protocol/sync.pb.h"
@@ -39,7 +39,7 @@ SYNC_EXPORT_PRIVATE void BuildNormalDownloadUpdates(
 // Helper function.  Defined here for testing.
 SYNC_EXPORT_PRIVATE void BuildNormalDownloadUpdatesImpl(
     ModelTypeSet proto_request_types,
-    UpdateHandlerMap* update_handler_map,
+    UpdaterList* updater_list,
     const sessions::NudgeTracker& nudge_tracker,
     sync_pb::GetUpdatesMessage* get_updates);
 
@@ -56,7 +56,7 @@ SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForConfigure(
 // Helper function.  Defined here for testing.
 SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForConfigureImpl(
     ModelTypeSet proto_request_types,
-    UpdateHandlerMap* update_handler_map,
+    UpdaterList* updater_list,
     sync_pb::GetUpdatesCallerInfo::GetUpdatesSource source,
     sync_pb::GetUpdatesMessage* get_updates);
 
@@ -72,7 +72,7 @@ SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForPoll(
 // Helper function.  Defined here for testing.
 SYNC_EXPORT_PRIVATE void BuildDownloadUpdatesForPollImpl(
     ModelTypeSet proto_request_types,
-    UpdateHandlerMap* update_handler_map,
+    UpdaterList* updater_list,
     sync_pb::GetUpdatesMessage* get_updates);
 
 // Sends the specified message to the server and stores the response in a member
@@ -87,7 +87,7 @@ SYNC_EXPORT_PRIVATE SyncerError
 SYNC_EXPORT_PRIVATE SyncerError ProcessResponse(
     const sync_pb::GetUpdatesResponse& gu_response,
     ModelTypeSet proto_request_types,
-    UpdateHandlerMap* handler_map,
+    UpdaterList* updater_list,
     sessions::StatusController* status);
 
 // Helper function to copy client debug info from debug_info_getter to