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This CL adds a flag that can be used to load an extension as login
screen extension during API tests.

Bug: 957573
Change-Id: Iba6299721df654d66856c7a24a4292f82938063b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599177
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664567}