Drive-by change: Add "at" method to ZoneChunkList.

R=petermarshall@chromium.org

Bug: v8:7754
Change-Id: I75f4e3f786640f2a53b467aab18abe01b4f5b360
Reviewed-on: https://chromium-review.googlesource.com/1144823Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#54569}