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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=caseq@chromium.org

Change-Id: Ic1010c21616f2349627b9219907376b419d85871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526941
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825403}