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This is the first step in replacing ScopedObserver. The new class is
functionally identical, but has slightly more descriptive member and
class names. Now also with a test.

Bug: 1145565
Change-Id: I5416d7873396238c829fcf31a55cfe0ef5bac1f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519763
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824539}