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This replaces Callback to OnceCallback, so that users of this can pass
OnceCallback.

Change-Id: Icc4b8633fb9aa1a6c49ab06583cae985f54df597
Reviewed-on: https://chromium-review.googlesource.com/930445Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539010}