[Sync] Add history delete directive type

sync/internal_api/public/base/model_type.h

Index: sync/internal_api/public/base/model_type.h
diff --git a/sync/internal_api/public/base/model_type.h b/sync/internal_api/public/base/model_type.h
index af528f82bf0205e5e7c01e5085505d8774a306ba..f743bff80b2333c77e00219b5510af998a10b22d 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -81,7 +81,9 @@ enum ModelType {
   EXTENSION_SETTINGS,
   // App notifications.
   APP_NOTIFICATIONS,
-  LAST_USER_MODEL_TYPE = APP_NOTIFICATIONS,
+  // History delete directives.
+  HISTORY_DELETE_DIRECTIVES,
+  LAST_USER_MODEL_TYPE = HISTORY_DELETE_DIRECTIVES,
 
   // An object representing a set of Nigori keys.
   NIGORI,
@@ -135,6 +137,9 @@ bool ShouldMaintainPosition(ModelType model_type);
 // preference flag, so not all of them are individually user-selectable.
 ModelTypeSet UserTypes();
 
+// This is the subset of UserTypes() that can be encrypted.
+ModelTypeSet EncryptableUserTypes();
+
 // Returns a list of all control types.
 //
 // The control types are intended to contain metadata nodes that are essential