[Sync] Add support for proxy types

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 d8a53486ef826e23bee2e5fbea43ddb64335f876..16d10480cad3f706213f5825a631efe1199a801f 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -148,6 +148,11 @@ SYNC_EXPORT ModelTypeSet UserTypes();
 // This is the subset of UserTypes() that can be encrypted.
 SYNC_EXPORT_PRIVATE ModelTypeSet EncryptableUserTypes();
 
+// Virtual types are placeholder types for handling implicitly enabling real
+// types. They do not exist at the server, and are simply used for
+// UI/Configuration logic.
+SYNC_EXPORT ModelTypeSet VirtualTypes();
+
 // Returns a list of all control types.
 //
 // The control types are intended to contain metadata nodes that are essential