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- SigninClient duplicates API that is already in
  SigninManagerBase::Observer. This CL removes these functions from the
  SigninClient and uses SigninManagerBase::Observer instead.
- Several SigninClient implementations depend on SigninErrorController,
  but this dependency is conceptually wrong. This CL moves the
  corresponding code out of SigninClient implementations.

Change-Id: I4a09e40a204713ba3cb8ea76e65c46228fc3923d
Bug: 836212
Reviewed-on: https://chromium-review.googlesource.com/c/1346950
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612170}