Cpp bindings: add typedef for callbacks.

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:
   static const char* Name_;
 
-  typedef {{interface.name}}Proxy Proxy_;
-  typedef {{interface.name}}Stub Stub_;
+  using Proxy_ = {{interface.name}}Proxy;
+  using Stub_ = {{interface.name}}Stub;
 
-  typedef {{interface.name}}RequestValidator RequestValidator_;
+  using RequestValidator_ = {{interface.name}}RequestValidator;
 {%- if interface|has_callbacks %}
-  typedef {{interface.name}}ResponseValidator ResponseValidator_;
+  using ResponseValidator_ = {{interface.name}}ResponseValidator;
 {%- else %}
-  typedef mojo::PassThroughFilter ResponseValidator_;
+  using ResponseValidator_ = mojo::PassThroughFilter;
 {%- endif %}
 {% if interface.client %}
-  typedef {{interface.client}} Client;
+  using Client = {{interface.client}};
 {% else %}
-  typedef mojo::NoInterface Client;
+  using Client = mojo::NoInterface;
 {% endif %}
 
 {#--- Constants #}
 {%- for constant in interface.constants %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}};
 {%- endfor %}
 
 {#--- Enums #}
 {%- for enum in interface.enums %}
 {%    macro enum_def() %}{% include "enum_declaration.tmpl" %}{% endmacro %}
   {{enum_def()|indent(2)}}
 {%- endfor %}
 
 {#--- Methods #}
   virtual ~{{interface.name}}() {}
 
 {%- for method in interface.methods %}
+{%    if method.response_parameters != None %}
+  using {{method.name}}Callback = {{interface_macros.declare_callback(method)}};
+{%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };