Mojo: Add support for InterfacePtr as a struct member

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 %}
     Serialize_(mojo::internal::Forward(input->{{pf.field.name}}), buf, &result->{{pf.field.name}}.ptr);
+{%-   elif pf.field.kind|is_interface_kind %}
+    result->{{pf.field.name}} = input->{{pf.field.name}}.PassMessagePipe().release();
 {%-   elif pf.field.kind|is_handle_kind %}
     result->{{pf.field.name}} = input->{{pf.field.name}}.release();
 {%-   else %}
     result->{{pf.field.name}} = input->{{pf.field.name}};
 {%-   endif %}
 {%- endfor %}
     *output = result;
   } else {
     *output = NULL;
   }
 }
 
 void Deserialize_(internal::{{struct.name}}_Data* input,
                   {{struct.name}}Ptr* output) {
   if (input) {
     {{struct.name}}Ptr result({{struct.name}}::New());
 {%- for pf in struct.packed.packed_fields %}
 {%-   if pf.field.kind|is_object_kind %}
     Deserialize_(input->{{pf.field.name}}.ptr, &result->{{pf.field.name}});
+{%-   elif pf.field.kind|is_interface_kind %}
+    if (input->{{pf.field.name}}.is_valid())
+      result->{{pf.field.name}}.Bind(mojo::MakeScopedHandle(mojo::internal::FetchAndReset(&input->{{pf.field.name}})));
 {%-   elif pf.field.kind|is_handle_kind %}
     result->{{pf.field.name}}.reset(mojo::internal::FetchAndReset(&input->{{pf.field.name}}));
 {%-   elif pf.field.kind|is_enum_kind %}
     result->{{pf.field.name}} = static_cast<{{pf.field.kind|cpp_wrapper_type}}>(
       input->{{pf.field.name}});
 {%-   else %}
     result->{{pf.field.name}} = input->{{pf.field.name}};
 {%-   endif %}
 {%- endfor %}
     *output = result.Pass();
   } else {
     output->reset();
   }
 }