[New go generator] Implement declaring unions.

mojom/generators/go/templates/templates.go

Index: mojom/generators/go/templates/templates.go
diff --git a/mojom/generators/go/templates/templates.go b/mojom/generators/go/templates/templates.go
index fd725af0d305aec6ebf1b66bb08b48c4756a5b1a..3de97ec1b54bff07ea274ee42a77f65b20569941 100644
--- a/mojom/generators/go/templates/templates.go
+++ b/mojom/generators/go/templates/templates.go
@@ -34,6 +34,7 @@ func ExecuteTemplates(tmplFile *translator.TmplFile) string {
 func init() {
 	// We parse the subtemplates only once.
 	goFileTmpl = template.New("GoFileTemplate")
+
 	template.Must(goFileTmpl.Parse(goFileTemplate))
 	template.Must(goFileTmpl.Parse(nonNullableFieldDecodingTmplText))
 	template.Must(goFileTmpl.Parse(fieldDecodingTmplText))
@@ -44,6 +45,11 @@ func init() {
 	template.Must(goFileTmpl.Parse(structDeclTmplText))
 	template.Must(goFileTmpl.Parse(structVersions))
 	template.Must(goFileTmpl.Parse(structDecodingTmplText))
+
+	template.Must(goFileTmpl.Parse(unionDeclTmplText))
+	template.Must(goFileTmpl.Parse(unionInterfaceDeclTmplText))
+	template.Must(goFileTmpl.Parse(unionFieldDeclTmplText))
+	template.Must(goFileTmpl.Parse(unknownUnionFieldDeclTmplText))
 }
 
 const goFileTemplate = `