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Also makes Subscription movable.

I'll need this to convert some existing code over to CallbackList.

Bug: none
Change-Id: Iad1c20f8d4facacea67cf4379e6bdb77e7984d1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343954
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796684}