For c++, Generate enum classes instead of enum from mojom.

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

 {#  TODO(yzshen): Make these templates more readable. #}
 
 {#  Computes the serialized size for the specified struct.
     |struct| is the struct definition.
     |input_field_pattern| should be a pattern that contains one string
     placeholder, for example, "input->%s", "p_%s". The placeholder will be
     substituted with struct field names to refer to the input fields.
     This macro is expanded to compute seriailized size for both:
     - user-defined structs: the input is an instance of the corresponding struct
       wrapper class.
@@ skipping 77 matching lines @@
 {%-     if not kind|is_nullable_kind %}
   MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING(
 {%-       if kind|is_interface_kind %}
       !{{output}}->{{name}}.handle.is_valid(),
 {%-       else %}
       !{{output}}->{{name}}.is_valid(),
 {%-       endif %}
       mojo::internal::VALIDATION_ERROR_UNEXPECTED_INVALID_HANDLE,
       "invalid {{name}} in {{struct_display_name}}");
 {%-     endif %}
+{%-   elif kind|is_enum_kind %}
+  {{output}}->{{name}} =
+    static_cast<int32_t>({{input_field}});
 {%-   else %}
   {{output}}->{{name}} = {{input_field}};
 {%-   endif %}
 {%- endfor %}
 {%- endmacro -%}
 
 {#  Deserializes the specified struct.
     |struct| is the struct definition.
     |input| is the name of the input struct instance.
     |output_field_pattern| should be a pattern that contains one string
@@ skipping 42 matching lines @@
 {%-     elif kind|is_any_handle_kind %}
     {{output_field}}.reset(mojo::internal::FetchAndReset(&{{input}}->{{name}}));
 {%-     elif kind|is_enum_kind %}
     {{output_field}} = static_cast<{{kind|cpp_wrapper_type}}>({{input}}->{{name}});
 {%-     else %}
     {{output_field}} = {{input}}->{{name}};
 {%-     endif %}
 {%-   endfor %}
   } while (false);
 {%- endmacro %}