sync: Add interfaces for per-type sync

sync/sessions/model_type_registry.h

Index: sync/sessions/model_type_registry.h
diff --git a/sync/sessions/model_type_registry.h b/sync/sessions/model_type_registry.h
index 126d05b834ae7dd7ebc99248065a779e34d2be2b..ad34ada962b69b2e3d4795a9f693452942daaeaf 100644
--- a/sync/sessions/model_type_registry.h
+++ b/sync/sessions/model_type_registry.h
@@ -20,12 +20,11 @@ namespace syncable {
 class Directory;
 }  // namespace syncable
 
-class SyncDirectoryUpdateHandler;
-class SyncDirectoryCommitContributor;
+class UpdateHandler;
+class CommitContributor;
 
-typedef std::map<ModelType, SyncDirectoryUpdateHandler*> UpdateHandlerMap;
-typedef std::map<ModelType, SyncDirectoryCommitContributor*>
-    CommitContributorMap;
+typedef std::map<ModelType, UpdateHandler*> UpdateHandlerMap;
+typedef std::map<ModelType, CommitContributor*> CommitContributorMap;
 
 // Keeps track of the sets of active update handlers and commit contributors.
 class SYNC_EXPORT_PRIVATE ModelTypeRegistry {