GitLab

Design doc:
go/app-service-instance-registry-design-doc
go/appservice-for-per-app-time-limit-design-doc

This CL adds the App Service Instance classes:
Instance
InstanceUpdate
InstanceRegistry

InstanceRegistry depends on Instance and InstanceUpdate to calculate
the instance updates to notify the client.

BUG=1011235

Change-Id: Ife2a25c1ae24b3a5c8e1ef67540ba49939126174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1896484Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712915}