GitLab

* The service shouldn't unregister from the topics, when handler
the Handler is unregistering from topic (as stated in the documentation
for for UnregisterHandler).
* Sync shouldn't unregister on browser shutdown

TBR=gab@chromium.org, pavely@chromium.org

Bug: 894752, 801985
Change-Id: If676d159ba7d7beeebf325bce2e78b5847da3f76
Reviewed-on: https://chromium-review.googlesource.com/c/1277908Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599206}