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..34e1df2f58a30568c8314569574061d32f4b4b82 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 +315,11 @@ 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 will be updated during configuration cycles, but otherwise

[tim (not reviewing), 2014/01/08 19:33:42]

Perhaps saying "updated when routing info changes, such as when entering or
exiting a configuration cycle" (or "changes when entering or exiting a
configuration cycle" if you want to avoid "routing info").

[rlarocque, 2014/01/09 00:31:18]

Done.  I went with the second suggestion.

+  // remains constant.
+  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_;