The fields are private, so prepend them with "_".
Drive-by: Replace std::map by std::unordered_map and avoid one
redundant lookup in this map.

R=titzer@chromium.org

Change-Id: Id4aad9bb36ec84daf581a79852d56895fc05429d
Reviewed-on: https://chromium-review.googlesource.com/877882
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50742}