Retire ScopedObserver in /chrome/browser/translate.

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=droger@chromium.org

Bug: 1145565
Change-Id: I6c7fe4f73ad87ba4db420266e183ec571da500db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2883562


Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880990}