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Move ownership of |ArcAuthContext| from |ArcSessionManager| to
|ArcBackgroundAuthCodeFetcher|.

ARC++ assumed that only one account will be synced between Chrome OS and
ARC++, i.e. the Device Account. This assumption is no longer true
because of the introduction of Chrome OS Account Manager (Design Doc is
linked in the bug id).
Syncing multiple accounts requires the ability to create multiple auth
code fetchers which in turn require multiple |ArcAuthContext|s. This was
not possible currently because of the ownership of a single
|ArcAuthContext| by |ArcSessionManager|.

|ArcSessionManager| does not really need an |ArcAuthContext|. It was
needed only for:

  - Passing the context to |ArcBackgroundAuthCodeFetcher|: Not required
  if |ArcBackgroundAuthCodeFetcher| creates and owns the context.
  - Getting an instance of Token Service: Can be retrieved directly via
  |ProfileOAuth2TokenServiceFactory|.
  - |account_id| and |full_account_id|: Can be retrieved directly via
  |SigninManagerBase|.

Bug: 871690
Test: browser_tests --gtest_filter="*ArcAuthServiceTest*"
Change-Id: I78a6d2aa6693529f67f480967258c9fb00469e02
Reviewed-on: https://chromium-review.googlesource.com/1171237
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585456}