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..aaaae53d53e8b252f7371b78932e874e42d1ae74 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 mostly-static state that affects the way we interact with

[Nicolas Zea, 2014/01/04 02:09:34]

elaborate on what "mostly" means?

[rlarocque, 2014/01/06 20:03:33]

Done.  It changes during configures, but is otherwise static.

+  // different sync types.
+  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_;