Now that they contain more than just one member variable, this might make
sense (and at least doesn't do any harm).

BUG=v8:6092

Change-Id: Ibd150bedf542580f69946f62f8aca47bce945499
Reviewed-on: https://chromium-review.googlesource.com/623748
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47547}