(Pre-work for adding special-purpose data structures as subclasses.)

BUG=v8:7308

Change-Id: I82a5a96e18c3321e9d5b783642516ff869b45f27
Reviewed-on: https://chromium-review.googlesource.com/1084474
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53482}