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Crashes are reported around getting root window in UpdateShelf(),
however the new AshPopupAlignmentDelegate is mostly same as
old WorkAreaObserver. One difference is the location of getting
root window. This could fix the problem.

If this doesn't fix the problem, then the problem happens outside
of this file.

BUG=400492
R=stevenjb@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/441013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287859 0039d316-1c4b-4281-b951-d872f2087c98