Improve Mojo permission service and chrome::PermissionManager observing.

This is fixing two bugs:
- mojo callbacks were not always run, which is now required.
- the service was not always unsubscribing from the PermissionManager,
  keeping stalled subscriptions alive.

BUG=430238, 437770

Review URL: https://codereview.chromium.org/1054653002

Cr-Commit-Position: refs/heads/master@{#323368}
5 files changed