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Following UX feedback, we add an animation that runs during deep
scanning.

Screencast: https://screencast.googleplex.com/cast/NjI3ODYxNjQ2OTk5NTUyMHwzMTZhNTk3YS02ZA
Fixed: 1076092
Change-Id: I1b12fcb88409740842bf5aeed6d5927ab94533bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367115
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801603}