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Starts tracking when a desk switch animation starts. Sets up a timer
which then records the number of traversals during a sequence of
desk animations. There may be multiple animations during the timer
duration, they will notify the metrics collector class on animation
end with how many traversals have happened.

For touchpad swipes, add some logic to track how much visible desk
switches the user has seen. This is different from the number of desk
activations. For all other desk switch sources, the number of
traversals during an animation is 1 + #Replacements.

Test: manual, added tests
Bug: 1146479
Change-Id: Ie3c46ebe80e35a768a89eb6e7105d807db087529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523061
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826426}