Adds removing:
  - references to sources in observer during sources destruction
    explicitly,
  - references to observer in sources during observer destruction
    using scoped observation.

Previously this logic was missing potentially leading to dangling
references in outliving sources/observer depending on the order of
destruction.

Bug: 40258491
Change-Id: I650fe6aa6b12c53d89be0e4e63521d9a9849ef06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5937909


Auto-Submit: Piotr Wrótniak <piotrwrotniak@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Piotr Wrótniak <piotrwrotniak@google.com>
Cr-Commit-Position: refs/heads/main@{#1378964}