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This changes the API to look like CallbackList's: instead of calling
UnregisterEvent(), callers destroy a subscription object they get from
registering.  This is a bit easier for callers to deal with.  However,
the necessity of also synchronously clearing the event from the WaitSet
makes the implementation a bit irritating, since while CallbackList does
provide a hook to be called back on callback removal, that happens after
notification is finished, rather than right at subscription destruction.
Instead of using the hook, wrap the CallbackList::Subscriptions in an
object that will also remove the WaitSet item if need be.

Bug: none
Change-Id: Ideb74b1435bfaad1647324e5ba5f90976c268e2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343956
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#796728}