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This CL is a part of moving access token management to
OAuth2AccessTokenManager.

It is specifically a step toward folding
DeviceO2TSDelegate into DeviceO2TS now that the latter
is no longer an O2TS subclass.

It moves GetRobotAccountId(), CheckRobotAccountId(),
OnServiceAccountIdentityChanged(), and
|service_account_identity_subscription_| which are
related to the robot account ID.

Bug: 967598
Change-Id: I63ffc5c9ee6f56bdadfdaf47b5b2eb870c77187f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694901
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676342}