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This moves users of ScopedObserver to either base::ScopedObservation
or base::ScopedMultiSourceObservation in the performance_manager.

Bug: 1145565
Change-Id: Ide766fc0139134ebc6f9cdfff58864fccb1b78f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520246
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824563}