Notification images and icons are downscaled down on the renderer main thread. This contributes excessive jank.

On Android, Notifications.LoadScaleDownTime.Image has a 50% percentile of 300 ms, a 90% percentile of 1000ms, and a 99% percentile of 3000ms.

Even on desktop, the 90% percentile is 250ms and 99% percentile is 1000ms.

(These numbers are partially mitigated by the fact that notifications arrive in the background, so this jank will only be visible if Chrome is in the foreground and the foreground tab shares a renderer process with the Service Worker displaying the notification. However that's not so unlikely, since users often spend a lot of time on websites that they grant notifications permission to).

The rescaling should be moved to a background thread. See Todos at:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/notifications/NotificationImageLoader.cpp?l=79&rcl=37402096ff8038dd57754bfd6f41fac15cff89df
and
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/notifications/NotificationImageLoader.cpp?l=84&rcl=37402096ff8038dd57754bfd6f41fac15cff89df