Intent to prototype and ship: Document.activeViewTransition

[Emilio Cobos Álvarez]

Hi,

In bug 2001836 I'm intending to land a minor extension to the view
tranisitions API that exposes the active view transition.

This is used in the native code all over the place, and it is convenient
for authors, see https://github.com/w3c/csswg-drafts/issues/12407.

Specification:
https://drafts.csswg.org/css-view-transitions-2/#dom-document-activeviewtransition
Preference: Behind the existing (default on) dom.viewTransitions.enabled
DevTools bug: N/A, shouldn't need special devtools consideration.
Extensions bug: N/A, same
Link to standards-positions discussion: N/A, really minor tweak to
existing view transitions API which we implement.
Other browsers:
* WebKit: Implemented
(https://bugs.webkit.org/show_bug.cgi?id=297255), can't check Safari
shipping status r/n.
* Blink: Shipping
(https://groups.google.com/a/chromium.org/g/blink-dev/c/ET1U6rt1Ihc/m/V2n0rvi5AgAJ).

Thanks,
-- Emilio