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This interface is basically a simpler VoteConsumer. It allows
to observe votes made through a voting channel without having
to deal with AcceptedVotes.

One limitation it has is that it is no longer possible to
submit multiple votes to the same context through a single
VotingChannel. In practice, this was never done anyways. To
support multiple votes for one context, it is easy to build
multiple voting channels.

Of course, AcceptedVotes still exists. Those are managed by
the VoteConsumerDefaultImpl, which is a temporary class that
handles creating and deleting AcceptedVotes as needed, then
notifying the VoteObserver.

The goal in the future is to fold this functionality into
the VotingChannelFactory and remove VoteConsumer entirely.

This will be done after each users of VoteConsumer are migrated
one at a time.

Bug: 971272
Change-Id: I4e890435cf29d4375ccb41b393b6d80b2d392778
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519207
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825971}