Develop IdentityManager::Observer equivalent to GCMS::Observer::OnAddAccountToCookieCompleted

CL adds an equivalent hook to
GaiaCookieManagerService::Observer::OnAddAccountToCookieCompleted
in IdentityManager::Observer.

This is part of the effort to allow GCMS::Observer be replaced by
IdentityManager::Observer. No functionality changes expected.

The unittests aim at ensuring the hook is called through
Observers of IdentityManager in case of successful or failing session
merges.

BUG=903718

Change-Id: Id08c4515171f3c21e134c2c129593e8977bd7e11
Reviewed-on: https://chromium-review.googlesource.com/c/1376529
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616713}
3 files changed