Go bindings: Change generated enum value constants to be typed as the enum type.

mojo/public/tools/bindings/generators/go_templates/enum.tmpl

 // Copyright 2015 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.
 
 {% import "runtime_type_macros.tmpl" as runtime_type_macros %}
 
 {% macro define(enum, typepkg, package) %}
 type {{enum|name}} int32
 
 const (
 {% for field in enum.fields %}
-{%   if field.value %}
+{%   if loop.first %}
+{%     if field.value %}
+        {{enum|name}}_{{field|name}} {{enum|name}} = {{field.value|expression_to_text}}
+{%     else %}
+        {{enum|name}}_{{field|name}} {{enum|name}} = 0
+{%     endif %}
+{%   elif field.value %}
         {{enum|name}}_{{field|name}} = {{field.value|expression_to_text}}
-{%   elif loop.first %}
-        {{enum|name}}_{{field|name}} = 0
 {%   else %}
         {{enum|name}}_{{field|name}} = {{enum|name}}_{{enum.fields[loop.index0 - 1]|name}} + 1;
-{%   endif %}
+{%   endif%}
 {% endfor %}
 )
 
 {%- set type_name = enum|name(exported) %}
 {{runtime_type_macros.maybeWriteStaticMojomTypeAccessor(typepkg, enum)}}
 {{runtime_type_macros.maybeWriteMojomTypeAccessor(typepkg, enum,type_name)}}
 
 {% endmacro %}