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This change migrates Callbacks in extensions::Blocklist to OnceCallbacks.

Bug: 1007786
Change-Id: If463e8b8fb776bfd1b5fdaae5b415c0216f6419e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425863
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809855}