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The shelf size can change, and the hotseat will jump if the bounds are
only calculated in reference to the shelf.

To fix this vertical jumping when going from in-app -> home, calculate the
hotseat y position in reference to the bottom of the display.

Bug: 1008455
Change-Id: Idd24dc17b0c53126be42d2638508e728f35f683d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1828074
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700888}