Mojo cpp bindings: add support for validating incoming messages.

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 
 {%- macro pass_params(parameters) %}
 {%-   for param in parameters %}
 {%-     if param.kind|is_object_kind -%}
 mojo::internal::Wrap(params->{{param.name}}())
 {%-     elif param.kind|is_interface_kind -%}
 mojo::MakeProxy<{{param.kind.name}}>(mojo::MakePassable(params->{{param.name}}()).Pass())
@@ skipping 218 matching lines @@
       return true;
 {%-     else %}
       break;
 {%-     endif %}
     }
 {%-   endfor %}
   }
 {%- endif %}
   return false;
 }
+
+{#--- Request validator definitions #}
+
+{{class_name}}RequestValidator::{{class_name}}RequestValidator(
+    mojo::MessageReceiver* next) : MessageValidator(next) {
+}
+
+bool {{class_name}}RequestValidator::Accept(mojo::Message* message) {
+  // TODO(yzshen): implement validation logic.
+  return next_->Accept(message);
+}
+
+{#--- Response validator definitions #}
+{% if interface|has_callbacks %}
+{{class_name}}ResponseValidator::{{class_name}}ResponseValidator(
+    mojo::MessageReceiver* next) : MessageValidator(next) {
+}
+
+bool {{class_name}}ResponseValidator::Accept(mojo::Message* message) {
+  // TODO(yzshen): implement validation logic.
+  return next_->Accept(message);
+}
+{%- endif -%}