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When an external display is attached a new RootWindowController and new
shelf are created for that display. Shelf alignment is loaded from prefs.
However, if the external display is marked as primary then the internal
and external displays are swapped. This means that the (new) shelf now
on the internal display never reloads its alignment from prefs. Since
the screen was locked at attach time, the shelf stays in the
bottom_locked state and never becomes visible.

Fix this by reloaded shelf alignment from prefs on display swap.

Bug: 748291
Test: added to ash_unittests
Change-Id: I5780dd3ca3b796b6f46fea9bc46fe875d123701e
Reviewed-on: https://chromium-review.googlesource.com/597410
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491452}