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It can observe IdentityManager::Observer instead. This is a step
forward in the migration to IdentityManager API.

Most usages of SigninManager were also replaced by their equivalent
IdentityManager API. It's also observing
IdentityManager::DiagnosticsObserver from now on.

BUG: 809539

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I9e7319a522f298298ab6a1a512bad73675050e70
Reviewed-on: https://chromium-review.googlesource.com/c/1276606Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#609640}