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This CL prepares for the upcoming usage of Gaia-keyed URL lookups in
Weblayer by turning SafeBrowsingTokenFetcher into an interface and its
implementation into a SafeBrowsingPrimaryAccountTokenFetcher subclass.
This layer of abstraction is needed as WebLayer will not fetch access
tokens via IdentityManager as the current implementation does.

A followup CL will abstract the remaining dependences on
//components/signin from safe_browsing_token_fetcher.h.

Bug: 1080748
Change-Id: Ib9c0cb63a3e1364611c21a3cc069bab7bdba0e4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613968
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841435}