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Also changes NotifiesOnlyOnChangesWithPreviousValue to pass
the previous value by const-ref.

Bug: 999594
Change-Id: I63ce2cdb1d085ac68a7abb2a7b8211be0a353c56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787727
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697810}