Mojo C++ bindings: support passing associated interface pointers/requests in method parameter lists…

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy
     : public {{interface.name}},
       public mojo::internal::ControlMessageProxy {
  public:
   explicit {{interface.name}}Proxy(mojo::MessageReceiverWithResponder* receiver);
 
 {%- for method in interface.methods %}
   void {{method.name}}(
       {{interface_macros.declare_request_params("", method)}}
   ) override;
 {%- endfor %}
+
+  mojo::internal::SerializationContext* serialization_context() {
+    return &serialization_context_;
+  }
+
+ private:
+  mojo::internal::SerializationContext serialization_context_;
 };