If an image's source changes to/from an animated gif we need to fire an ANIMATED state change. I don't know if listening for a src attribute change is enough here since it might still need to load the image to know if it is animated. A better indicator is nsAccessibilityService::NotifyOfImageSizeAvailable, but we will need to store the previous state for that.

Changing severity to S3 because this is ongoing cache the world work that does not affect current users.

Use imgINotificationObserver to get notified of size availability.
Layout uses an observer to wait for this too. Our observer is notified
after layout so we should have bounds by then.

We need to store the request status because we get a lot of "replayed"
status changes that would cause chatty and wrong state change events
when the accessible is first created and bound to parent.

We can use that status for both INVISIBLE and ANIMATED states in
NativeState.

Depends on D127718

Depends on D127719

Backed out for causing hybrid build bustages

• Backout link
• Push with failures
• Failure Log

Backed out for Bp-hybrid bustages on ImageAccessible

Backout link: https://hg.mozilla.org/integration/autoland/rev/acc023ae7f611c778481eb6bce33c2f319e9fc10
Log link: https://treeherder.mozilla.org/logviewer?job_id=354415748&repo=autoland&lineNumber=84327

https://hg.mozilla.org/mozilla-central/rev/30572d7dd9dd
https://hg.mozilla.org/mozilla-central/rev/0133dadaf3eb
https://hg.mozilla.org/mozilla-central/rev/10f92eac8bc7