File Manager Extension: Stop using quirky values in element.animate()
File Manager Extension: Stop using quirky values in element.animate()

This change avoids us using quirky CSS values in calls to element.animate().
By specifying length values with "px" units we avoid using quirky
dimensionless CSS values on dimension properties.

BUG=526921

Review URL: https://codereview.chromium.org/1314423004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#346579}

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