[mojo] Add pickling support for native-only structs

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

 {%- from "enum_macros.tmpl" import is_valid_enum_def %}
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   {{is_valid_enum_def(enum, class_name=struct.name)}}
 {%- endfor %}
 
 // static
 {{struct.name}}Ptr {{struct.name}}::New() {
   {{struct.name}}Ptr rv;
@@ skipping 19 matching lines @@
 {%-     else %}
   rv->{{field.name}} = {{field.name}};
 {%-     endif %}
 {%-   endfor %}
   return rv.Pass();
 }
 {%  endif %}
 
 bool {{struct.name}}::Equals(const {{struct.name}}& other) const {
 {%-  for field in struct.fields %}
+{%-    if field.kind|is_typemapped_kind %}
+{#     TODO(rockot): Consider adding support for native equality testing. #}
+  NOTREACHED() << "Equality testing not supported with typemapped fields.";
+{%-    else %}
   if (!mojo::internal::ValueTraits<{{field.kind|cpp_wrapper_type}}>::Equals(this->{{field.name}}, other.{{field.name}}))
     return false;
+{%-    endif %}
 {%-  endfor %}
   return true;
 }