[Sync] Support commit only types.

components/sync/base/model_type.h

Index: components/sync/base/model_type.h
diff --git a/components/sync/base/model_type.h b/components/sync/base/model_type.h
index 1d84ceda90400bc1d740c8949501b3a3c45a9ba7..d091d1b4fa3c375518380e605368303c8f902df7 100644
--- a/components/sync/base/model_type.h
+++ b/components/sync/base/model_type.h
@@ -245,6 +245,9 @@ ModelTypeSet CoreTypes();
 // Those core types that have high priority (includes ControlTypes()).
 ModelTypeSet PriorityCoreTypes();
 
+// Types that may commit data, but should never be included in a GetUpdates.
+ModelTypeSet CommitOnlyTypes();
+
 // Determine a model type from the field number of its associated
 // EntitySpecifics field.  Returns UNSPECIFIED if the field number is
 // not recognized.