They can have unboxed double fields and embedder fields.

Bug: chromium:775055
Change-Id: Idff67c776cb4209d78006b8f3f8ebc07aa509c42
Reviewed-on: https://chromium-review.googlesource.com/723425Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48655}