mojom: Add associative arrays to the mojom language.

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

 size_t GetSerializedSize_(const {{struct.name}}Ptr& input) {
   if (!input)
     return 0;
   size_t size = sizeof(internal::{{struct.name}}_Data);
 {%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
   size += GetSerializedSize_(input->{{pf.field.name}});
 {%- endfor %}
   return size;
 }
 
 void Serialize_({{struct.name}}Ptr input, mojo::internal::Buffer* buf,
                 internal::{{struct.name}}_Data** output) {
   if (input) {
     internal::{{struct.name}}_Data* result =
         internal::{{struct.name}}_Data::New(buf);
 {%- for pf in struct.packed.packed_fields %}
 {%-   if pf.field.kind|is_object_kind %}
 {%-     if pf.field.kind|is_any_array_kind %}
     mojo::SerializeArray_<{{pf.field.kind|get_array_validate_params|indent(26)}}>(
         mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
+{%-     elif pf.field.kind|is_map_kind %}
+    mojo::SerializeMap_<{{pf.field.kind.value_kind|get_map_validate_params|indent(26)}}>(
+        mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
 {%-     else %}
     Serialize_(mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
 {%-     endif %}
 {%-     if not pf.field.kind|is_nullable_kind %}
     MOJO_INTERNAL_DLOG_SERIALIZATION_WARNING(
         !result->{{pf.field.name}}.ptr,
         mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
         "null {{pf.field.name}} field in {{struct.name}} struct");
 {%-     endif %}
 {%-   elif pf.field.kind|is_any_handle_kind %}
@@ skipping 35 matching lines @@
       input->{{pf.field.name}});
 {%-   else %}
     result->{{pf.field.name}} = input->{{pf.field.name}};
 {%-   endif %}
 {%- endfor %}
     *output = result.Pass();
   } else {
     output->reset();
   }
 }