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This makes it easier to track down and/or change types.

Also renames CallbackList -> RepeatingCallbackList.

Bug: 1113007
Change-Id: Ib934071a88a991178dee8a99f722770f4e3f3957
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354614Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798495}