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This avoids instantiating a real InfoBarService in tests that don't need it,
which will in turn avoid constructing platform-specific InfoBar instances, which
will in turn prevent crashes in an upcoming change where I make the
views-specific implementation rely on objects these tests don't create.

This also addresses my followup comments on
https://chromium-review.googlesource.com/c/chromium/src/+/953711 .

BUG=none
TEST=none

Change-Id: Iab027693b982db7b91a3be8681647a112ba86e9e
Reviewed-on: https://chromium-review.googlesource.com/989213Reviewed-by: Patti <patricialor@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547904}