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Notify shelf config observers that the shelf config has changed when
tablet mode changes - some shelf dimensions depend on tablet mode
state, so shelf may need relayout when tablet mode changes. This is
generally done when hiding shelf buttons in tablet mode is enabled, but
depending on user settings, that may not be the case.

Also, stop observing tablet mode state in ShelfLayoutManager, as it's
already observing both ShelfConfig, so now that shelf config is updated
when tablet mode changes, it should be enough to observe shelf config
only.

BUG=None

Change-Id: I3c9f84ec9fa4d03bcd2838ea43cd7ae5869f26a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283580Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785847}