GitLab

This CL adds SubscriptionManager interface and renames the current
implementation to SubscriptionManagerImpl.

Bug: 745383
Change-Id: I1cbbb2b77e987a1d300f5926ae5bef8d20839dc1
Reviewed-on: https://chromium-review.googlesource.com/576027Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487462}