[Draft] try out the idea of adding export macros for C++ bindings.

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

 {% from "enum_macros.tmpl" import enum_decl -%}
 
 class {{struct.name}}_Reader;
 
-class {{struct.name}} {
+class {{dll_export}} {{struct.name}} {
  public:
   using Data_ = internal::{{struct.name}}_Data;
   using Reader = {{struct.name}}_Reader;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   {{enum_decl(enum)|indent(2)}}
 {%- endfor %}
 
 {#--- Constants #}
@@ skipping 25 matching lines @@
 {%- endif %}
   bool Equals(const {{struct.name}}& other) const;
 
 {#--- Struct members #}
 {%  for field in struct.fields %}
 {%-   set type = field.kind|cpp_wrapper_type %}
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 };