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The new ServiceDirectory can be used to publish services for other
processes to consumer.
Also refactored GN templates in fuchsia-sdk to allow building FIDL
interfaces defined outside of the SDK and added a unittest that verifies
that a service published in ServicesDirectory can be consumed in
ComponentContext.

TBR=jam@chromium.org

Bug: 831384
Change-Id: Ie82e8cd6ff7c02a791ec8b5534f55a527b08a571
Reviewed-on: https://chromium-review.googlesource.com/1027126
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555215}