[s13n] Replace inheritance of PO2TS::Observer by IdentityManager::Observer in AboutSigninInternals

CL is an incremental step to get away from PO2TS from AboutSigninInternals.

BUG=809539

Change-Id: Id8e8baea53c76eedee1bb3ac83f2ccb347909210
Reviewed-on: https://chromium-review.googlesource.com/c/1426760
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#625167}
2 files changed