Add FlushForTesting to InterfacePtr and Binding.

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

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