Mojo C++ bindings: support mapping mojo string to WTF::String.

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 %}
 
 class {{interface.name}} {
@@ skipping 37 matching lines @@
 {%    if method.response_parameters != None %}
 {%-     if method.sync %}
   virtual bool {{method.name}}({{interface_macros.declare_sync_method_params("", method)}}) {
     // Sync method. This signature is used by the client side; the service side
     // should implement the signature with callback below.
     NOTREACHED();
     return false;
   }
 {%-     endif %}
 
-  using {{method.name}}Callback = {{interface_macros.declare_callback(method)}};
+  using {{method.name}}Callback = {{interface_macros.declare_callback(method, use_wtf_types)}};
 {%-   endif %}
   virtual void {{method.name}}({{interface_macros.declare_request_params("", method)}}) = 0;
 {%- endfor %}
 };