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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=msramek@chromium.org

Change-Id: I118ba2a88ecbbbe731ff98061c4e11c6255abcb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2525764
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825438}