sync: Introduce ModelTypeRegistry and helpers

sync/internal_api/sync_manager_impl.h

Index: sync/internal_api/sync_manager_impl.h
diff --git a/sync/internal_api/sync_manager_impl.h b/sync/internal_api/sync_manager_impl.h
index 5d5cfaab552bc96d5f3f5dd14cdffc2a754bf570..ae79ba70955ab8d881b9836366b04c6dc37dc000 100644
--- a/sync/internal_api/sync_manager_impl.h
+++ b/sync/internal_api/sync_manager_impl.h
@@ -34,6 +34,7 @@ namespace syncer {
 class SyncAPIServerConnectionManager;
 class WriteNode;
 class WriteTransaction;
+class ModelTypeRegistry;
 
 namespace sessions {
 class SyncSessionContext;
@@ -314,6 +315,10 @@ class SYNC_EXPORT_PRIVATE SyncManagerImpl :
   // client (the Syncer) and the sync server.
   scoped_ptr<SyncAPIServerConnectionManager> connection_manager_;
 
+  // Maintains state that affects the way we interact with different sync types.
+  // This state changes when entering or exiting a configuration cycle.
+  scoped_ptr<ModelTypeRegistry> model_type_registry_;
+
   // A container of various bits of information used by the SyncScheduler to
   // create SyncSessions.  Must outlive the SyncScheduler.
   scoped_ptr<sessions::SyncSessionContext> session_context_;