[ios] Expose more event through TabsDependencyInstaller

Some of the sub-classes of TabsDependencyInstaller wants to observe
the change of the active WebState or the final destruction of a tab
(to perform some cleanup).

Expose this via two new methods of TabsDependnecyInstaller to limit
the number of code that has to observe WebStateList (i.e. without
this change, the sub-classes would have to observe WebStateList
twice).

Bug: 409299519
Change-Id: If91837e99309eb6b0a1143678ffc0a6ed8bed304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6708543
Reviewed-by: Federica Germinario <fedegermi@google.com>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1483638}