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This CL creates a mediator for connecting TabStrip to WebStateList and
as for now the TabStrip shows the number of tabs open.

Bug: 1136834,1128249
Change-Id: I9082b675f88a30002f9113a363b1b1e7c2b62465
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461010
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822673}