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Update AshPopupAlignmentDelegate calculations of its work area.

When windows are docked this reduces the work area for windows.
AshPopupAlignmentDelegate uses the work area to determine the positioning of
notifications. Update this calculation to include the docked region.

TEST=AshPopupAlignmentDelegateTest.DockedWindow
BUG=284574

Review URL: https://codereview.chromium.org/1121893004

Cr-Commit-Position: refs/heads/master@{#329486}