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In a previous CL [1], we made changes to the shelf layout manager to
use occluded and displaced bounds. However, we introduced a bug where
the work area would not reset when we hide the accessibility keyboard,
leaving a large gap in the screen.

The code now makes more sense: we should be changing the work area
whenever the displaced bounds change.

We add a test for this. We also removed some calls to |LayoutShelf| in
existing tests; |LayoutShelf| doesn't get called in real situations.
Calling |LayoutShelf| manually will always change the work area,
which is not what happens in practice.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1146530

TBR=jamescook@chromium.org

Bug: 870503
Change-Id: I80a6aa90f1bce8e59e6c7cde2e3f181c31fc6907
Reviewed-on: https://chromium-review.googlesource.com/1160827Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580394}