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Add tests for layout of users list after rotation and after
virtual keyboard apperance changes.

Extract LoginKeyboardTestBase class as a base for tests that
need to manipulate virtual keyboard.

Bug: 807827
Change-Id: I46a300ebbce8a59c9648573fede203ca0f21a292
Reviewed-on: https://chromium-review.googlesource.com/896966
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534115}