R=titzer@chromium.org

Bug: v8:7754, v8:7965
Change-Id: I5cb76b4e989fbe6d4d08bafb60dbbb26c3d58e2d
Reviewed-on: https://chromium-review.googlesource.com/1156515Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54820}