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IdentityProvider is deprecated as a general-purpose means of interacting
with the user's Google accounts. This CL ports ExtensionDownloader
away from it, having it instead take in the OAuth2TokenService directly
as well as a callback that returns the account to use with the webstore.
It also clarifies the lifetime relationship between ExtensionDownloader
and the ProfileOAuth2TokenService/SigninManager instances on which it
depends for authentication.

In the long term ExtensionDownloader will be ported to interact with the
Identity Service for its use case. This CL is an incremental step on the
path that also fills the more near-term goal of eliminating usages of
IdentityProvider.

Bug: 809966, 809927
Change-Id: I6b82cc318bd807d559e1701391ed3e02fcb489aa
Reviewed-on: https://chromium-review.googlesource.com/1051810
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561024}