Bug 1779808 - Add a JS helper to check if a document is the current active document of a browsing context, r=emilio

This should get tested as part of bug 1692468 and others.

This is exposing https://searchfox.org/mozilla-central/rev/99c3ccc2f02c5fff7c9850f7a0770ee92cc693f6/dom/base/Document.cpp#12730 to privileged JS.

If one wants to check if an element in a data document (document.implementation.createHTMLDocument().body for example) is in a window which is active,
something like element.ownerGlobal?.document.isActive() should work.

Differential Revision: https://phabricator.services.mozilla.com/D151951

dom/webidl/Document.webidl

@@ -741,3 +741,10 @@ partial interface Document {
   [ChromeOnly]
   Wireframe? getWireframe(optional boolean aIncludeNodes = false);
 };
+
+partial interface Document {
+  // Returns true if the document is the current active document in a browsing
+  // context which isn't in bfcache.
+  [ChromeOnly]
+  boolean isActive();
+};