Organizing model type, notification type, root tag and model type string in map.

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 383135b13fd731fd4255d4443a0036c02daa15ed..5d353fdec9a237581423c7fc92295a700a99621e 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -36,6 +36,10 @@ namespace syncer {
 
 // A Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.sync
+
+// |kModelTypeInfoMap| struct entries are in the same order as their definition
+// in ModelType enum. Don't forget to update the |kModelTypeInfoMap| struct in
+// model_type.cc when you make changes in ModelType enum.
 enum ModelType {
   // Object type unknown.  Objects may transition through
   // the unknown state during their initial creation, before