We should probably discourage random access given that it isn't
constant time for this data structure. You can always still do it
via the Find() function if you really need to - at least the weird
interface tells you that something strange is going on.

Change-Id: I5e20cf9172afaa9265f1a6e38f619543b65614f2
Reviewed-on: https://chromium-review.googlesource.com/1184916Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55317}