[New go generator] Implement declaring unions.

mojom/generators/go/translator/mojom_file.go

Index: mojom/generators/go/translator/mojom_file.go
diff --git a/mojom/generators/go/translator/mojom_file.go b/mojom/generators/go/translator/mojom_file.go
index d4de4846e8db7d5d3b0ca07d76a9221f2b024b22..c352728df227e8889e3dbefd61af3201982f7e3a 100644
--- a/mojom/generators/go/translator/mojom_file.go
+++ b/mojom/generators/go/translator/mojom_file.go
@@ -10,6 +10,7 @@ type TmplFile struct {
 	PackageName string
 	Imports     []Import
 	Structs     []*StructTemplate
+	Unions      []*UnionTemplate
 }
 
 type Import struct {
@@ -69,6 +70,28 @@ type structVersion struct {
 	Version uint32
 }
 
+type UnionTemplate struct {
+	// Name is the name of the union in go code.
+	Name string
+
+	// Fields contains the list of fields of the union.
+	Fields []UnionFieldTemplate
+}
+
+type UnionFieldTemplate struct {
+	// Name is the name of the field in go.
+	Name string
+
+	// Type is the go type of the field value.
+	Type string
+
+	// Tag of the field.
+	Tag uint32
+
+	// Union is the union containing this field.
+	Union *UnionTemplate
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 // EncodingInfo describes the information necessary to encode a field.