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

Also renames CallbackList -> RepeatingCallbackList.

Bug: 1113007
Change-Id: I42357489f2c3e7a19b0f4909fe20ca7dc9ff4339
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354225Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797771}