This hides more implementation details and simplifies callers.

R=ahaas@chromium.org

Bug: 
Change-Id: I4809611c55b810a3b0674713e12f3f17401e6c9c
Reviewed-on: https://chromium-review.googlesource.com/620713Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47434}