Oops it should be start in clamshell mode and switch to tablet.

The opposite happens when we start in tablet. On switching to clamshell, the first item in overflow shows up on the main shelf now that we have enough space
since no back button. But the item on overflow does not disappear as expected.

+mccanny kejiashao

Should we just hide the overflow bubble or re-layout so that the right items appear?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0df97e8caf7282eaabdef6c02c6afede76637b14

commit 0df97e8caf7282eaabdef6c02c6afede76637b14
Author: Sammie Quon <sammiequon@google.com>
Date: Sat Jul 14 02:38:46 2018

shelf: Update overflow bubble when enter exit tablet mode.

Depending on if we start in tablet or clamshell, there can either be a
shelf item shown on neither shelf, or a shelf item shown on both shelfs.

Test: added test
Bug:  863169 
Change-Id: Id99c3f270d49db8acea535b689576c3de077ea0c
Reviewed-on: https://chromium-review.googlesource.com/1135751
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575137}
[modify] https://crrev.com/0df97e8caf7282eaabdef6c02c6afede76637b14/ash/shelf/shelf_view.cc
[modify] https://crrev.com/0df97e8caf7282eaabdef6c02c6afede76637b14/ash/shelf/shelf_view_unittest.cc