GitLab

Make notification_registrar_ shared between old and new implementations.
No behaviour change - feature is hidden behind the flag.

Drive by: Remove unused header in bookmark_app_installation_task.cc

Bug: None
Change-Id: I4e1f735a60a2befe61f427741d3db820dd528091
Reviewed-on: https://chromium-review.googlesource.com/c/1370241Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615718}