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Sync would like to use one AccessTokenFetcher instance to
fetch access tokens both for the primary account and for secondary
accounts. For the primary account, they would like to wait for the
refresh token to be available. In order to support this use case,
AccessTokenFetcher needs a Mode parameter. This CL adds that Mode
parameter.

Note that this Mode parameter is *not identical* semantically to the
Mode parameter of PrimaryAccountAccessTokenFetcher: the former's
|kWaitUntilAvailable| mode makes an access token request once there
is an account that is both (a) primary and (b) has a refresh token
available, while the latter's new |kWaitUntilRefreshTokenAvailable|
mode simply waits until there is a refresh token available for the
given account (which may no longer be primary at that point, even if
it was at the time of creating the AccessTokenFetcher). This semantic
distinction is called out explicitly in the comments on
PrimaryAccountAccessTokenFetcher::Mode. Sync has indicated that the
AccessTokenFetcher semantics suffice for their use case.

Bug: 840703, 729547
Change-Id: Iff39a24ef74945b3e9fa22ad961c3cd73a401e51
Reviewed-on: https://chromium-review.googlesource.com/1158831
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582535}