I tried to use more specific union types where appropriate, even though
many of these fields are accessed as Object from C++.

Bug: v8:7793
Change-Id: I771d9b6459bdc1413019f8ff5ddfd611d1adf61f
Reviewed-on: https://chromium-review.googlesource.com/c/1490573
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59975}