Mojo cpp bindings: add support for validating incoming messages.

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 class {{interface.name}}Proxy;
 class {{interface.name}}Stub;
+
+class {{interface.name}}RequestValidator;
+{%- if interface|has_callbacks %}
+class {{interface.name}}ResponseValidator;
+{%- endif %}
 {% if interface.client %}
 class {{interface.client}};
 {% endif %}
 
 class {{interface.name}} {
  public:
   typedef {{interface.name}}Proxy Proxy_;
   typedef {{interface.name}}Stub Stub_;
+
+  typedef {{interface.name}}RequestValidator RequestValidator_;
+{%- if interface|has_callbacks %}
+  typedef {{interface.name}}ResponseValidator ResponseValidator_;
+{%- else %}
+  typedef mojo::PassThroughFilter ResponseValidator_;
+{%- endif %}
 {% if interface.client %}
   typedef {{interface.client}} Client_;
 {% else %}
   typedef mojo::NoInterface Client_;
 {% endif %}
 
 {#--- Enums #}
 {%- for enum in interface.enums %}
 {%    macro enum_def() %}{% include "enum_declaration.tmpl" %}{% endmacro %}
   {{enum_def()|indent(2)}}
 {%- endfor %}
 
 {#--- Methods #}
   virtual ~{{interface.name}}() {}
+
 {%- if interface.client %}
   // Called once before any other method.
   virtual void SetClient({{interface.client}}* client) = 0;
 {%- else %}
   virtual void SetClient(mojo::NoInterface* client) {}
 {%- endif %}
 {%- for method in interface.methods %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };