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Adds necessary observation code and plumbing to the tab switcher view
controller to add UI in the future.

Bug: none
Change-Id: I5912042ecf4634b7a8c0b3628b40505293cfcdaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536454
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830672}