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Adds a single-client equivalent of ScopedServiceBinding<>, useful for
e.g. Agents which public separate services to each connected component,
and expect that there will be at most one connection to each service by
the component.

Bug: 920920
Change-Id: Id7b527fb2e636bd9ef54290a0ac8d947d60a51b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574652Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665595}