GitLab

This CL converts SyncFileMetadataCallback and SyncFileStatusCallback to
OnceCallbacks.

It also deletes the unused SyncFileSetCallback.

Bug: 1152272
Change-Id: Ib85b776c602cdffb871f79ad158f0726c7ed9594
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2606603
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840953}