GitLab

- Properly suffixes BigInt constants with 'n'
- Fixes stringification of constants into valid .js syntax
- Improve default values handling in structs which was only partially
  implemented previously

Bug: 883496
Change-Id: Id270d2f34dd40fa970d58448c1002c691db35a87
Reviewed-on: https://chromium-review.googlesource.com/c/1321577
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606529}