sync: Introduce control data 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 20c8ffa19d0a37d860e9170c706d7e58dca0bcba..818d0bf4ccd0b82a6580f6473519f9c34d148187 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -124,6 +124,19 @@ SYNC_EXPORT ModelType GetModelTypeFromSpecifics(
 // value (sibling ordering) for this item.
 bool ShouldMaintainPosition(ModelType model_type);
 
+// If this returns true, this type belongs in GROUP_CONTROL
+//
+// The control types are always implicitly enabled, since they contain metadata
+// which may affect the processing of other data types.
+//
+// The syncer has special code to deal with the encryption, update application
+// and conflict resolution of these types.  They will not be processed by the
+// usual set of syncer update application and conflict resolution routines.
+bool IsControlType(ModelType model_type);
+
+// A complete list of all control types.
+ModelTypeSet ControlTypes();
+
 // Determine a model type from the field number of its associated
 // EntitySpecifics field.
 ModelType GetModelTypeFromSpecificsFieldNumber(int field_number);