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It had some logic for counting window under observation, but
CheckedObserver is a better way of catching use-after-free errors.

Bug: none
Test: bots
Change-Id: I21020c194a848c21e434ad0e8583684053b1f663
Reviewed-on: https://chromium-review.googlesource.com/c/1315991
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605092}