Port AvatarToolbarButton away from GaiaCookieManagerService callbacks

It was observing the gaia cookie manager service to know whether the
list of gaia accounts in the cookie jar has changed. That can be
achieved observing IdentityManager as well.

Bug: 921008
Change-Id: Icad5a14f1288346b97d4d8ccc1eb4c0a901eae3b
Reviewed-on: https://chromium-review.googlesource.com/c/1408928
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623187}
2 files changed