Generate a templated Clone method for all mojo structs.

mojo/public/interfaces/bindings/tests/struct_with_traits.mojom

Index: mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
diff --git a/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom b/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
index 460003a470e90075922b4f063f24ef138dd3daeb..7bbb4336e92a0d8a48437f8e54de1882aa2651c6 100644
--- a/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
+++ b/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
@@ -19,6 +19,10 @@ struct StructWithTraits {
   array<NestedStructWithTraits> f_struct_array;
 };
 
+struct StructWithTraitsContainer {
+  StructWithTraits f_struct;
+};
+
 struct PassByValueStructWithTraits {
   handle f_handle;
 };