Mojo C++ bindings templates: use the same templates for user-defined structs and method params.

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::internal::Buffer* buf) {
-    return new (buf->Allocate(sizeof({{class_name}})))
-        {{class_name}}();
-  }
-
-  static bool Validate(const void* data,
-                       mojo::internal::BoundsChecker* bounds_checker) {
-    {{struct_macros.validate(struct)|indent(2)}}
-  }
-
-  mojo::internal::StructHeader header_;
-{{struct_macros.fields(struct)}}
-
-  void EncodePointersAndHandles(std::vector<mojo::Handle>* handles) {
-    {{struct_macros.encodes(struct)|indent(2)}}
-  }
-
-  void DecodePointersAndHandles(std::vector<mojo::Handle>* handles) {
-    {{struct_macros.decodes(struct)|indent(2)}}
-  }
-
- private:
-  {{class_name}}() {
-    header_.num_bytes = sizeof(*this);
-    header_.version = {{struct.versions[-1].version}};
-  }
-};
-static_assert(sizeof({{class_name}}) == {{struct.versions[-1].num_bytes}},
-              "Bad sizeof({{class_name}})");