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

It moves RegisterTokenResponse to private since it's
only used by OAuth2AccessTokenManager::Fetcher which is
a nested class in OAuth2AccessTokenManager.

Bug: 967598
Change-Id: Ic51db4d938f955e920a33d00a6f74e07f88a8763
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731029Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#683106}