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As usual, its callers must be made async as well (except in tests),
all the way back up to the nearest PostTask.

To avoid too many PostTasks, its methods now accept batches of ids.

Bug: 1142591

Change-Id: I53be2948dbe46ab020da1cd096ee59b775ac842c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569779
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835228}