Make SigninManager no longer a keyed service.

After https://crrev.com/c/1514792 SigninManager is never used as a
keyed service.

Bug: 939372
Change-Id: Ib51cfc195e15752c45b57a3f497314adb0e7431d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529062
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642500}
5 files changed