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Minor refactor that shortens and clarifies the callback queuing mechanism.

Change-Id: Ibebb44009ad7a4091d7df2c25fdcba2f174d0489
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506362
Commit-Queue: Olivier Yiptong <oyiptong@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822756}