GitLab

Creates / destroys TabLifecycleUnits and notifies observers as tabs are
created / destroyed.

Bug: 775644
Change-Id: I8042e656c2672dbc71ad0187c151140a098f1356
Reviewed-on: https://chromium-review.googlesource.com/789410
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523844}