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This completes ScopedObserver updates for Windows, Mac and Android.
iOS is the final holdout.

Bug: 998625
Change-Id: I415fe2f6799fcd1631385c4a825027424d8bfc0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854520Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705735}