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One of IdentityProvider's clients' use cases is to fetch access tokens
for the active account. IdentityProvider currently provides no explicit
mechanism for this, instead letting clients directly interact with the
underlying OAuth2TokenService instance via GetTokenService(). However,
it's necessary to decouple IdentityProvider from OAuth2TokenService at
an interface level in order to allow for porting ProfileIdentityProvider
to be backed by IdentityManager.

This CL adds an explicit interface for clients of IdentityProvider to
request access tokens for the active account. This interface is
currently implemented directly in IdentityProvider itself, as it is
common to ProfileIdentityProvider and DeviceIdentityProvider. Before
porting ProfileIdentityProvider to be backed by IdentityManager we
will move this implementation out into those two derived classes,
temporarily duplicating it. However, we will only do that when we are
just at the point of porting ProfileIdentityProvider away from using
ProfileOAuth2TokenService.

Bug: 809452
Change-Id: I402af0cb19b5e0057edc3870ca2a8f564f4f8e10
Reviewed-on: https://chromium-review.googlesource.com/1089051Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565584}