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As described in the subsection "Thumb Strip to Tab grid transition" of
the Thumb Strip Design Doc (go/bling-thumb-strip-design), in order to
make a transition of layout in the tab switcher when the user goes from
thumb strip to tab grid, we create the layout switcher and layout
switcher provider, which allow the ViewRevealingVerticalPanHandler to
trigger a layout transition in the grid view controller.

The grid view controller implements the layout switcher protocol, and
the tab grid view controller implements the layout switcher provider
protocol. The tab grid coordinator connects the
viewRevealingVerticalPanHandler to the layout switcher provider.

Bug: 1127604, 1130037
Change-Id: Ibb170f6f1e69b70c0b20a6dd4dbff3daa507187c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2408012Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Roberto Moura <mouraroberto@google.com>
Cr-Commit-Position: refs/heads/master@{#809278}