Bug 1705289 use more restrictive variable declarations r=jib

No behavior changes.

Depends on D123559

Differential Revision: https://phabricator.services.mozilla.com/D123560

dom/media/webrtc/tests/mochitests/test_getUserMedia_basicWindowshare.html

@@ -15,8 +15,8 @@
    * cycle for an screenshare MediaStream on a video HTMLMediaElement.
    */
   runTest(async function () {
-    var testVideo = createMediaElement('video', 'testVideo');
-    var constraints = {
+    const testVideo = createMediaElement('video', 'testVideo');
+    const constraints = {
       video: {
          mozMediaSource: "window",
          mediaSource: "window"