SigninManagerBase: Change AddObserver to SetObserver.

IdentityManager is intended to be the sole observer of
SigninManagerBase, so we shouldn't allow multiple observers.

Bug: 939372
Change-Id: I81454f4174e19c1dad8fe38df4191878fa9512e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1515573
Commit-Queue: Lowell Manners <lowell@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640289}
6 files changed