Mojo C++ bindings: support enum validation.

mojo/public/tools/bindings/generators/cpp_templates/struct_declaration.tmpl

 {%- set class_name = struct.name ~ "_Data" -%}
 
 class {{class_name}} {
  public:
   static {{class_name}}* New(mojo::internal::Buffer* buf);
 
   static bool Validate(const void* data,
                        mojo::internal::BoundsChecker* bounds_checker);
 
   void EncodePointersAndHandles(std::vector<mojo::Handle>* handles);
   void DecodePointersAndHandles(std::vector<mojo::Handle>* handles);
 
+{% from "enum_macros.tmpl" import enum_data_decl -%}
+{#--- Enums #}
+{%- for enum in struct.enums -%}
+  {{enum_data_decl(enum)|indent(2)}}
+{%- endfor %}
+
   mojo::internal::StructHeader header_;
 {%- for packed_field in struct.packed.packed_fields %}
 {%-   set name = packed_field.field.name %}
 {%-   set kind = packed_field.field.kind %}
 {%-   if kind.spec == 'b' %}
   uint8_t {{name}} : 1;
-{%-   elif kind|is_enum_kind %}
-  int32_t {{name}};
 {%-   else %}
   {{kind|cpp_field_type}} {{name}};
 {%-   endif %}
 {%-   if not loop.last %}
 {%-     set next_pf = struct.packed.packed_fields[loop.index0 + 1] %}
 {%-     set pad = next_pf.offset - (packed_field.offset + packed_field.size) %}
 {%-     if pad > 0 %}
   uint8_t pad{{loop.index0}}_[{{pad}}];
 {%-     endif %}
 {%-   endif %}
 {%- endfor %}
 
 {%- set num_fields = struct.versions[-1].num_fields %}
 {%- if num_fields > 0 %}
 {%-   set last_field = struct.packed.packed_fields[num_fields - 1] %}
 {%-   set offset = last_field.offset + last_field.size %}
 {%-   set pad = offset|get_pad(8) %}
 {%-   if pad > 0 %}
   uint8_t padfinal_[{{pad}}];
 {%-   endif %}
 {%- endif %}
 
  private:
   {{class_name}}();
   ~{{class_name}}() = delete;
 };
 static_assert(sizeof({{class_name}}) == {{struct.versions[-1].num_bytes}},
               "Bad sizeof({{class_name}})");