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As part of this CL, we move LockScreen control and user account functions into
LoginTestBase and make LoginKeyboardTestBase a subclass. In the future, test
fixtures with new login-related functionality can inherit from LoginTestBase.

Bug: 809330
Change-Id: I0d082a60e667fec7fd391984de6469ead5d57e77
Reviewed-on: https://chromium-review.googlesource.com/c/1338476Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609363}