Retire ScopedObserver in /chrome/browser/profiles.

ScopedObserver is being deprecated in favor of two new classes:
- base::ScopedObservation for observers that only ever observe
  a single source.
- base::ScopedMultiSourceObservation for observers that do or may
  observe more than a single source.

This CL was uploaded by git cl split.

R=alexilin@chromium.org

Bug: 1145565
Change-Id: I66dfd191bdf56dfba0606d448e192dc6a0e6196b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2883889


Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880992}