Clean up functions that manage TabLifecycleObservers

TabLifecycleObserver is very similar to LifecycleUnitObserver, but
each observer is notified on changes to ANY LifecycleUnit, while a
LifecycleUnitObserver is notified on changes to a single
LifecycleUnit. This cleans up the TabLifecycleObserver code to make
it easier to merge the two classes in the future:

* Removes all knowledge of TabLifecycleObserver from the
  TabLifecycleUnit class. Instead TabLifecycleUnitSource becomes a
  LifecycleUnitObserver - whenever it gets a state change
  notification, it notifies all TabLifecycleObservers.
* Removes the redundant TabManager::AddObserver and
  TabLifecycleUnitExternal::AddTabLifecycleObserver methods: callers
  can use TabLifecycleUnitSource::AddTabLifecycleObserver directly.
* Adds ScopedObservationTraits so that existing code using
  ScopedObservation can switch to
  TabLifecycleUnitSource::AddTabLifecycleObserver.

A followup will change the list of TabLifecycleObservers in
TabLifecycleUnitSource to a list of LifecycleUnitObservers. Observers
passed to TabLifecycleUnitSource::AddTabLifecycleObserver will be
notified of changes to all lifecycle units, and observers passed to
LifecycleUnit::AddObserver will be notified of changes to that
specific unit.

R=aattar

Bug: 394889323
Change-Id: I2e26251b69fd5046693b7aa5122d4f8f9090405e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6287727
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Alex Attar <aattar@google.com>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1429205}