Improve CreateAccessTokenFetcher{,ForAccount,ForClient} APIs

Change OAuth2TokenService::CreateAccessTokenFetcher to return
a std::unique_ptr<...> instead of a raw pointer.

Mark CreateAccessTokenFetcher{,ForAccount,ForClient} method with
WARN_UNUSED_RESULT to ensure the newly created object is used.

Fix a unit test that was not using the fetcher.

Bug: none
Change-Id: I670ed6dea48f6d9a1bdf4e384370fe1eff68570a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663136
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670506}
21 files changed