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This class is for observers that observe only one source.
This is more readable and slightly more efficient than using
base::ScopedObserver, where an observer has zero or one subscriptions.

Change-Id: Iefd987ce006aa2106249cf2708f63d90326814ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518028Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824014}