Mojo C++ Bindings: Support custom impl ref types

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
+
+template <typename ImplRefTraits>
 class {{interface.name}}Stub;
 
 class {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
 class {{interface.name}}ResponseValidator;
 {%- endif %}
 
 class {{export_attribute}} {{interface.name}}
     : public {{interface.name}}InterfaceBase {
  public:
   static const char Name_[];
   static const uint32_t Version_ = {{interface.version}};
   static const bool PassesAssociatedKinds_ = {% if interface|passes_associated_kinds %}true{% else %}false{% endif %};
   static const bool HasSyncMethods_ = {% if interface|has_sync_methods %}true{% else %}false{% endif %};
 
   using Proxy_ = {{interface.name}}Proxy;
-  using Stub_ = {{interface.name}}Stub;
+
+  template <typename ImplRefTraits>
+  using Stub_ = {{interface.name}}Stub<ImplRefTraits>;
 
   using RequestValidator_ = {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
   using ResponseValidator_ = {{interface.name}}ResponseValidator;
 {%- else %}
   using ResponseValidator_ = mojo::PassThroughFilter;
 {%- endif %}
 
 {#--- Enums #}
 {%- for enum in interface.enums %}
@@ skipping 19 matching lines @@
   // should implement the signature with callback below.
   virtual bool {{method.name}}({{interface_macros.declare_sync_method_params("", method)}});
 {%-     endif %}
 
   using {{method.name}}Callback = {{interface_macros.declare_callback(method,
       for_blink, use_new_wrapper_types)}};
 {%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };