This prevent forgotten field initializations.

Bug: v8:5267
Change-Id: Iaef5f79af360e2400b29afe878f80870c8a64e71
Reviewed-on: https://chromium-review.googlesource.com/970241
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52049}