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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 HandleAccessTokenFetch in
OAuth2AccessTokenManagerTest to test that StartRequest
checks HandleAccessTokenFetch() from |delegate_|
before FetchOAuth2Token.

Bug: 967598
Change-Id: I48802ef0fd0b2c353040234a2ca105b7b73cda27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722363
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682164}