CrOS Shelf: Allow setting 'animate' bool when laying out shelf

Consolidate |LayoutShelf| and |LayoutShelfAndUpdateBounds|.

Bug: 1035722, 1034725, 1033233, 1035717
Change-Id: I3ea844a6d10f5d8d6f328680350ce76c056c3717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980528
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727426}