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After https://crrev.com/c/1669293, the public API of IdentityManager
lives in //components/signin/public/identity_manager. This CL moves the
classes that are only used by IdentityManager to
//components/signin/internal/identity_manager.

Design doc:
https://docs.google.com/document/d/1MiZmqvDFVcCOseAuViJqd8wLDILbykarX7PfwleNpAs

TBR=jochen@chromium.org

Bug: 952788
Change-Id: I6ae60dd68e727124fb580820a345f4732c89f983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695401
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677705}