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There are many code sites which calculates the work area insets for positioning
a tray bubble or pop-up notification. Now that the calculation is more complex,
it makes sense to consolidate everything to one function.

This CL also fixes insets for the message center bubble and lock screen in
tablet mode.

TEST=manually verified
BUG=1040693

Change-Id: Ic8381a0b749e9e95b56717a239285f4bbddf7f08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993746Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729990}