[Sync] Add support for proxy types

sync/syncable/model_type.cc

Index: sync/syncable/model_type.cc
diff --git a/sync/syncable/model_type.cc b/sync/syncable/model_type.cc
index 1b0bee44cc32ecc7731da35ca4b6b8444b71661a..8a741198f735943fe8328e544777d5b849b59f87 100644
--- a/sync/syncable/model_type.cc
+++ b/sync/syncable/model_type.cc
@@ -290,6 +290,10 @@ ModelTypeSet EncryptableUserTypes() {
   encryptable_user_types.Remove(HISTORY_DELETE_DIRECTIVES);
   // Synced notifications are not encrypted since the server must see changes.
   encryptable_user_types.Remove(SYNCED_NOTIFICATIONS);
+  // Virtual types have no sync representation and are therefore not encrypted
+  // either. Their underlying representations may or may not be encrypted
+  // though.
+  encryptable_user_types.RemoveAll(VirtualTypes());
   return encryptable_user_types;
 }
 
@@ -305,6 +309,12 @@ ModelTypeSet ControlTypes() {
   return set;
 }
 
+ModelTypeSet VirtualTypes() {
+  ModelTypeSet set;
+  // TODO(zea): add a TABS type here.
+  return set;
+}
+
 bool IsControlType(ModelType model_type) {
   return ControlTypes().Has(model_type);
 }