Fix delay property of notification transition (CSS)

Delay is time and time can't be unitless in CSS:

"Even if all units represent the same time for the value 0, the unit may
not be omitted in that case as it isn't a <length>: 0 is invalid and
does not represent 0s, 0ms."
https://developer.mozilla.org/en-US/docs/Web/CSS/time

This fixes the hiding transition in Firefox.

Change-Id: I4f4001897691ef860ae321bae3913d8338fb897f
Merge "Fix delay property of notification transition (CSS)"