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This CL is developed under a flag.
It adds an action sheet that asks for confirmation when 'Close All' button is tapped on the tab grid to avoid unwanted clearing
Metrics related to the "Close all" button have been moved in the mediator.

Bug: 1119319
Change-Id: Ie1901b7f24faf63745290310efef805d11859940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362925
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803414}