[New go generator] Implement declaring unions.

mojom/generators/go/templates/templates.go

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package templates
 
 import (
         "bytes"
         "text/template"
 
@@ skipping 16 matching lines @@
 
         if err != nil {
                 panic(err)
         }
         return src
 }
 
 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))
 
-        template.Must(goFileTmpl.Parse(nonNullableFieldEncodingTmplText))
-        template.Must(goFileTmpl.Parse(fieldEncodingTmplText))
-        template.Must(goFileTmpl.Parse(structEncodingTmplText))
-        template.Must(goFileTmpl.Parse(structDeclTmplText))
-        template.Must(goFileTmpl.Parse(structVersions))
-        template.Must(goFileTmpl.Parse(structDecodingTmplText))
+        initEncodingTemplates()
+        initDecodingTemplates()
+        initStructTemplates()
+        initUnionTemplates()
 }
 
 const goFileTemplate = `
 {{- define "FileTemplate" -}}
 {{- $fileTmpl := . -}}
 package {{$fileTmpl.PackageName}}
 
 import (
         {{range $import := $fileTmpl.Imports}}
         {{$import.PackageName}} "{{$import.PackagePath}}"
         {{end}}
 )
 
 {{- range $struct := $fileTmpl.Structs}}
-        {{ template "StructDecl" $struct }}
+        {{ template "Struct" $struct }}
+{{- end}}
 
-        {{ template "StructEncodingTmpl" $struct }}
-
-        {{ template "StructVersions" $struct }}
-
-        {{ template "StructDecodingTmpl" $struct }}
+{{- range $union := $fileTmpl.Unions}}
+        {{ template "Union" $union }}
 {{- end}}
 {{- end -}}
 `