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This CL adds SplitOnceCallback, a utility function that allows to get
two OnceCallbacks out of one. Invoking any of the two split callbacks
will run the originally passed callback. Invoking the remaining callback
will result in a no-op.

Bug: 1156809
Change-Id: Iebf4542732fb6230af92fa27c4d7c5705933ad6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586688
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838267}