Implement User Activation API

Rewrote internal user activation tree (spreading state to other elements)
logic to match HTML spec:
https://html.spec.whatwg.org/multipage/interaction.html#user-activation-processing-model

Added navigator.userActivation API to expose internal user activation.

Also fixed a WPT test to conform to spec (siblings are not activated),
see also spec issue: whatwg/html#9831

Co-authored-by: Tom Schuster <[email protected]>

Differential Revision: https://phabricator.services.mozilla.com/D185348

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791079
gecko-commit: fba49989dde6aa32244c622fd43abe19a32128ae
gecko-reviewers: dom-core, webidl, saschanaz, edgar

html/user-activation/propagation-sameorigin.html

@@ -62,9 +62,11 @@
                 assert_false(msg.hasBeenActive);
             }, "Grandchild frame initial state");
         } else if (msg.type == 'child-one-report') {
+            // Siblings (same or cross origin) should not be activated per spec
+            // Spec issue discussing: https://github.com/whatwg/html/issues/9831
             test(() => {
-                assert_true(msg.isActive);
-                assert_true(msg.hasBeenActive);
+                assert_false(msg.isActive);
+                assert_false(msg.hasBeenActive);
             }, "Child1 frame final state");
         } else if (msg.type == 'child-sameorigin-report') {
             // This msg was triggered by a user click.