Drive-by: Add some comments.

R=jarin@chromium.org, mtrofin@chromium.org
BUG=v8:6434

Change-Id: If4ee8dfc68685429a91885a1c831709a4e31619c
Reviewed-on: https://chromium-review.googlesource.com/513202
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45498}