R=titzer@chromium.org

Bug: v8:7754, v8:7965
Change-Id: I62027d97c40276b5dfa9579fc4e903ee21a55b54
Reviewed-on: https://chromium-review.googlesource.com/1160539
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54894}