Encode and decode fields of union type.

mojom/generators/go/templates/encoding.go

Index: mojom/generators/go/templates/encoding.go
diff --git a/mojom/generators/go/templates/encoding.go b/mojom/generators/go/templates/encoding.go
index 56c6235cd8573c8705f623c4aa2c134132d4a945..df63d819903c226e427b6664b9192e766eecd58e 100644
--- a/mojom/generators/go/templates/encoding.go
+++ b/mojom/generators/go/templates/encoding.go
@@ -11,7 +11,7 @@ import (
 const fieldEncodingTmplText = `
 {{- define "FieldEncodingTmpl" -}}
 {{- $info := . -}}
-{{- if $info.IsNullable -}}
+{{- if and (not $info.IsUnion) $info.IsNullable -}}
 if {{$info.Identifier}} == nil {
 {{- if $info.IsPointer -}}
 	encoder.WriteNullPointer()
@@ -51,6 +51,21 @@ if err := encoder.WriteHandle({{$info.Identifier}}); err != nil {
 if err := {{$info.Identifier}}.Encode(encoder); err != nil {
 	return err
 }
+{{- else if $info.IsUnion -}}
+{{- if $info.IsPointer -}}
+encoder.StartNestedUnion()
+{{end -}}
+if {{$info.Identifier}} == nil {
+{{- if $info.IsNullable -}}
+	encoder.WriteNullUnion()
+{{- else -}}
+	return &bindings.ValidationError{bindings.UnexpectedNullUnion, "unexpected null union"}
+{{- end -}}
+} else {
+	if err := {{$info.Identifier}}.Encode(encoder); err != nil {
+		return err
+	}
+}
 {{- else if $info.IsArray -}}
 {{ $elInfo := $info.ElementEncodingInfo -}}
 encoder.StartArray(uint32(len({{$info.Identifier}})), {{$elInfo.BitSize}})