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This CL renames variables that have obsolete names like "gaiaAuthHost",
now that the "GaiaAuthHost" class doesn't exist anymore and is finally
renamed to "Authenticator" everywhere (see crrev.com/c/1605919).

Also fixes some comments that were referring to the old "auth host"
thing.

This is a pure refactoring CL, no functional changes are expected.

Bug: 470893
Test: existing tests
Change-Id: Ia07803527e0bf9148e021266cb2e2bc9a1fa67ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617832Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661360}