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Avoid GetAllWidgets in classic ash and mus Chrome OS browser tests.
These configs cannot get a list of root windows at the Views layer.
Add a DCHECK to avoid using GetAllWidgets in those scenarios.

Win/Linux/Mac query their WMs directly for root windows; on Chrome OS:
1) Mash can use MusClient to get a list of root windows.
2) Unit tests can use AuraTestHelper to get the root window.
3) Non-mash browser tests must use ash::Shell to get root windows.

This should land after http://crrev.com/c/919922

Bug: 811940
Change-Id: I609531529ff5c4379f6c1de47ca878b30e7dd614
Reviewed-on: https://chromium-review.googlesource.com/922268Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537331}