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When the 'shelf-hotseat' flag is enabled, the back button will only be
shown in tablet mode when in-app.

Some unittests have been changed to ensure the back button functionality
still works both with and without the 'shelf-hotseat' flag enabled.

Bug: 1012043
Change-Id: I9754f7a3d08473d4ecdd1573aa1ecf6ce33186ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857186
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708054}