Repro'd on 71.0.3551.0

The delay is less than a second, but I guess the expectation is that it's 0 (or reasonably close to).

This comes from the WebUI Dialog code [1], which delays the hide for 500ms [2].

I'll see what I can do there.

[1] - https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/dialogs.js?l=39&rcl=a3be34098f115e8a6f9a91821de47e5fe8e8163e
[2] - https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/dialogs.js?l=282-291&rcl=a3be34098f115e8a6f9a91821de47e5fe8e8163e

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/39778d0895e5a1055f4d4f75e394596ea150985f

commit 39778d0895e5a1055f4d4f75e394596ea150985f
Author: Luciano Pacheco <lucmult@chromium.org>
Date: Tue Oct 02 02:41:21 2018

Change Dialog background color only when it's visible

This fixes the perception to users that the dialog is taking too long
to disappear.

Test: No change in behavior, so current test suite should cover it. Additionally, manually tested file manager file deletion confirmation.
Bug:  883659 
Change-Id: I414a7282f70a64c0826954ef850a16e0bc37182e
Reviewed-on: https://chromium-review.googlesource.com/1242725
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595703}
[modify] https://crrev.com/39778d0895e5a1055f4d4f75e394596ea150985f/ui/file_manager/file_manager/foreground/css/common.css