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This is useful for callers who never want to cancel callbacks and can
guarantee the lifetime requirements.  For example, callbacks bound on
the same object that owns the CallbackList.

Bug: 1113007
Change-Id: I258d029360213a689bf888cca662ae56dec4cd74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337537
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795349}