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

It is specifically a step toward folding O2TS into PO2TS
now that O2TS is only one subclass of PO2TS.

It introduces OAuth2AccessTokenManagerTest to test
OAuth2AccessTokenManager. It has a basic test for
OAuth2AccessTokenManager to test StartRequest. It's
similar to ProfileOAuth2TokenService except refresh
token.  It doesn't check refresh token because it
assumes it has already a valid refresh token.

On follow-up CLs, tests will be rearranged between
ProfileOAuth2TokenService and OAuth2AccessTokenManager.

Bug: 967598
Change-Id: I10dfda0b3922bdcf6ac9e7abf7297f6e5de5bfc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712945
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680393}