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According to the specification, when you drag a window to snap it, the
preview area (referred to as "Indicator") should fade out over 67 ms,
fast out, linear in:
https://mccanny.users.x20web.corp.google.com/www/splitscreen-motion/index.html#window-drop

Currently, it fades out over 250 ms, fast out, slow in.

BEFORE:
https://photos.app.goo.gl/MQa3rZ4jKX4hZLba7

AFTER:
https://photos.app.goo.gl/iLFHyvqxFZsfreEC6

Test: manual
Bug: 934977
Change-Id: I27d3e4d758097f2a87207880b2bc5ca9622dad7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504037
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638300}