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This is a step in the direction of letting each shelf component
calculate and store its target bounds instead of having everything in
the shelf layout manager.

This change doesn't modify any of the actual semantics of bound
calculation yet.

Change-Id: Ibf08b425e9799d97f93fc9a4b17156ed15ae34d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032566Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737168}