R=titzer@chromium.org

Bug: v8:7754, v8:7965
Change-Id: I810f362f57a690f890d962a88f80df14c7de65db
Reviewed-on: https://chromium-review.googlesource.com/1160226Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54861}