Properly re-add PermissionManager as a permissions::observer.

Fixes faulty logic that did not handle the edgecase of a type_count
with a count of 0 properly. This case happens when all subscriptions of
that type are removed. This change properly handles that edge case and
tests for it.

Bug: 1205546
Change-Id: I83e64db79a0949ac9c5cb9c9fdba88d4f8b9c1f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2886380


Commit-Queue: James Hollyer <jameshollyer@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882425}