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The shelf layout manager currently calculates and sets the bounds of
all the shelf-related widgets (navigation, hotseat, status area).

The bound calculation logic should remain in this class, but the
actual setting of the bounds and managing animations can be done by
each widget at their leisure without requiring a full relayout of all
other widgets. For that to happen, the shelf layout manager can be
the central place from which widgets can ask their target bounds, but
they can then be responsible for setting them themselves at their own
pace.

This CL makes the target bounds a field so that other classes can
query them at any time (through getters that will be added
subsequently as other classes need them).

This has no expected user-visible effect.

Bug: 1021698
Change-Id: I6214d21315aec42e01d44f9f8ace4b041e970704
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919603Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715900}