Retire ScopedObserver in /chrome/browser/content_settings.

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=dullweber@chromium.org

Bug: 1145565
Change-Id: I31b11f29f545fecc1bf5369b29c9fbf44d4bedcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2883695


Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880989}