Bug 1605207 - Fixup XDR of FieldInitializers. r=jandem

Move the transcoding from XDRLazyScript and XDRRelazificationInfo into
LazyScript::XDRScriptData to avoid duplication. Add similar code for non-lazy
scripts for consistency.

Depends on D57919

Differential Revision: https://phabricator.services.mozilla.com/D57920

--HG--
extra : moz-landing-system : lando