Mojo cpp bindings: fix |num_fields| in struct header of *Params_Data.

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set class_name = struct.name ~ "_Data" %}
 class {{class_name}} {
  public:
   static {{class_name}}* New(mojo::Buffer* buf) {
     return new (buf->Allocate(sizeof({{class_name}})))
         {{class_name}}();
   }
 
 {#--- Setters -#}
@@ skipping 14 matching lines @@
   }
 
   bool DecodePointersAndHandles(mojo::Message* message) {
     {{ struct_macros.decodes(struct)|indent(4) }}
     return true;
   }
 
  private:
   {{class_name}}() {
     _header_.num_bytes = sizeof(*this);
-    _header_.num_fields = 3;
+    _header_.num_fields = {{struct.packed.packed_fields|length}};
   }
 
   mojo::internal::StructHeader _header_;
 {{ struct_macros.fields(struct) }}
 };
 MOJO_COMPILE_ASSERT(sizeof({{class_name}}) == {{struct.packed|struct_size}},
                     bad_sizeof_{{class_name}});