[Sync] Start implementation of migration for USS.

components/sync/engine_impl/model_type_registry.h

Index: components/sync/engine_impl/model_type_registry.h
diff --git a/components/sync/engine_impl/model_type_registry.h b/components/sync/engine_impl/model_type_registry.h
index 9e4ce33e8a7be4550676bcf525528740c601be02..91861109c3a3eb0c8d34043eb2425cc48cb34605 100644
--- a/components/sync/engine_impl/model_type_registry.h
+++ b/components/sync/engine_impl/model_type_registry.h
@@ -20,6 +20,7 @@
 #include "components/sync/engine/non_blocking_sync_common.h"
 #include "components/sync/engine/sync_encryption_handler.h"
 #include "components/sync/engine_impl/nudge_handler.h"
+#include "components/sync/syncable/user_share.h"
 
 namespace syncer {
 
@@ -32,10 +33,6 @@ class ModelTypeWorker;
 class UpdateHandler;
 struct ModelTypeState;
 
-namespace syncable {
-class Directory;
-}  // namespace syncable
-
 typedef std::map<ModelType, UpdateHandler*> UpdateHandlerMap;
 typedef std::map<ModelType, CommitContributor*> CommitContributorMap;
 typedef std::map<ModelType, DirectoryTypeDebugInfoEmitter*>
@@ -47,7 +44,7 @@ class ModelTypeRegistry : public ModelTypeConnector,
  public:
   // Constructs a ModelTypeRegistry that supports directory types.
   ModelTypeRegistry(const std::vector<scoped_refptr<ModelSafeWorker>>& workers,
-                    syncable::Directory* directory,
+                    UserShare* user_share,
                     NudgeHandler* nudge_handler);
   ~ModelTypeRegistry() override;
 
@@ -111,6 +108,10 @@ class ModelTypeRegistry : public ModelTypeConnector,
   ModelTypeSet GetEnabledNonBlockingTypes() const;
   ModelTypeSet GetEnabledDirectoryTypes() const;
 
+  syncable::Directory* directory() const {
+    return user_share_->directory.get();
+  }
+
   // Sets of handlers and contributors.
   ScopedVector<DirectoryCommitContributor> directory_commit_contributors_;
   ScopedVector<DirectoryUpdateHandler> directory_update_handlers_;
@@ -132,8 +133,8 @@ class ModelTypeRegistry : public ModelTypeConnector,
   // The known ModelSafeWorkers.
   std::map<ModelSafeGroup, scoped_refptr<ModelSafeWorker>> workers_map_;
 
-  // The directory.  Not owned.
-  syncable::Directory* directory_;
+  // The user share. Not owned.
+  UserShare* user_share_;
 
   // A copy of the directory's most recent cryptographer.
   std::unique_ptr<Cryptographer> cryptographer_;