Add field-initializing constructors to generated mojo structs.

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

Index: mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl
index f4aa314fae9e1987503f7f92388102120e797809..a6b5f35ca760da396b0c3ba7f92f47bb9367d94a 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl
@@ -1,12 +1,11 @@
 template <typename StructPtrType>
 {{struct.name}}Ptr {{struct.name}}::Clone() const {
-  // Use StructPtrType to prevent the compiler from trying to compile this
-  // without being asked.
-  StructPtrType rv(New());
+  return New(
 {%- for field in struct.fields %}
-  rv->{{field.name}} = mojo::internal::Clone({{field.name}});
+      mojo::internal::Clone({{field.name}})
+{%-   if not loop.last -%},{%- endif %}
 {%- endfor %}
-  return rv;
+  );
 }
 
 template <typename T,