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This addresses nits from https://crrev.com/c/2518028, and generally
tides up ScopedObservation and ScopedMultiSourceObservation.

Bug: 1145565
Change-Id: I6aa515b25eede775b17a21b34f688e3daf8410a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533899Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826833}