Optimized tab model restoration.

When restoring tabs state, every tab model used to observe tab strip
model changes, and handler fired with N^2 frequency. With large number
of tabs this has a noticeable overhead even though there may be no real
done in event handlers - you just pay for numerous calls of
base::ObserverList.

The changeset replaces observing to direct calls which removes excessive
work while restoring tabs state.

Change-Id: I21cf252ca059d951d5674ca0d665dd9b4ffd1a9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8098227
Commit-Queue: Alexander Yashkin <a-v-y@yandex-team.ru>
Reviewed-by: Alison Gale <agale@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1667714}