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When disconnecting an external monitor the display manager shuts down
the associated RootWindowController, but defers deletion of the
RootWindowController via a posted task. This means an ash::Shelf
can exist without a ShelfWidget.

A similar code path exists for entering tablet mode, which turns on
display mirroring, and shuts down the RootWindowController for the
external display.

If there's a pending task that changes session state (for example,
the user's sign-in completes), then SessionController observers can
fire and attempt to update the alignment of the shelf.

Add null checks to ash::Shelf for shelf_widget_. I think we used to
have these many years ago, and I think I removed them in 2016 when
when cleaning up ash::Shelf. Sadly, they are still needed.

Bug: 937495, 738011
Test: ash_unittests
Change-Id: I794c610ba1f56483dae49f49e6d955ccf15ce10d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542572Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645321}