Bug 1590864 - part3 : use 'notifyUserGestureActivation()' to activate document directly. r=edgar

I found that this test sometime would fail on the try server, and the reason is that the child document doesn't be activated correctly even if we have called `synthesizeMouseAtCenter()`.

In order to reduce the dependency of other modules, which might affect synthesizing event, we should use `notifyUserGestureActivation` to ensure that we indeed activate the document.

Differential Revision: https://phabricator.services.mozilla.com/D51685

--HG--
extra : moz-landing-system : lando

dom/media/test/file_autoplay_policy_activation_frame.html

@@ -17,7 +17,7 @@
       window.addEventListener("message",
         (event) => {
           if (event.data == "click") {
-            synthesizeMouseAtCenter(document.body, {});
+            SpecialPowers.wrap(document).notifyUserGestureActivation();
           } else if (event.data == "play-audible") {
             playAndPostResult(false, event.source);
           } else if (event.data == "play-muted") {

dom/media/test/file_autoplay_policy_activation_window.html

@@ -50,7 +50,7 @@
             let test_case = event.data;
             // Click the window to activate if appropriate.
             if (test_case.activated_parent) {
-              synthesizeMouseAtCenter(document.body, {});
+              SpecialPowers.wrap(document).notifyUserGestureActivation();
             }
             let parent_window = event.source;
             if (test_case.same_origin_child === undefined) {