Minor fixes of things found in the code walkthrough.

R=titzer@chromium.org

Bug: v8:8015
Change-Id: If28be312fb7b8e5add214161a98d25145cdf8de5
Reviewed-on: https://chromium-review.googlesource.com/1224433
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55873}