cros: Dedup concurrent auth attempts on views-based lock screen.

If there are concurrent attempts (ie, spamming spacebar and enter) lock
would get into a bad state. Prevent concurrent attempts.

Bug: 719015
Change-Id: I4e40368fc8daa1d6ba3c47d4d181a0331b29d2a2
Reviewed-on: https://chromium-review.googlesource.com/688734
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521111}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/login_screen_controller_unittest.cc[diff]
• ash/login/mock_login_screen_client.cc[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/lock_screen_sanity_unittest.cc[diff]
• ash/login/ui/login_auth_user_view.cc[diff]
• ash/login/ui/login_auth_user_view.h[diff]
• ash/login/ui/login_password_view.cc[diff]
• ash/login/ui/login_password_view.h[diff]
• ash/login/ui/login_password_view_test.cc[diff]
• testing/buildbot/filters/ash_unittests_mash.filter[diff]