Bug 1760843 - P1 - add WPT for gDM frameRate:max;r=jib

Depends on D141937

Differential Revision: https://phabricator.services.mozilla.com/D142323

testing/web-platform/tests/screen-capture/getdisplaymedia.https.html

@@ -7,6 +7,7 @@
 <script src="/resources/testharnessreport.js"></script>
 <script src="/resources/testdriver.js"></script>
 <script src="/resources/testdriver-vendor.js"></script>
+<video id="display"></video>
 <script>
   'use strict';
 test(() => {
@@ -35,6 +36,25 @@ promise_test(t => {
     'getDisplayMedia should have returned an already-rejected promise.');
 }, `getDisplayMedia() must require user activation`);
 
+promise_test( async t => {
+  const v = document.getElementById('display');
+  v.autoplay = true;
+  // work around firefox bug 1586505, orthogonal to what's being tested
+  const frames = () => v.getVideoPlaybackQuality()?.totalVideoFrames || v.mozPaintedFrames;
+  const target_rate = 5;
+  const stream = await getDisplayMedia({video: {width:160, frameRate: target_rate}});
+  t.add_cleanup(() => stopTracks(stream));
+  v.srcObject = stream;
+  const intitial_time = v.currentTime;
+  const initial_frame_count = frames();
+  await new Promise(r => t.step_timeout(r, 10000));
+  const total_elapsed_frames = frames() - initial_frame_count;
+  const total_elapsed_time = v.currentTime - intitial_time;
+  const average_fps = total_elapsed_frames / total_elapsed_time;
+  assert_greater_than_equal(average_fps, target_rate * 0.50);
+  assert_less_than_equal(average_fps, target_rate * 1.75);
+}, "getDisplayMedia() must adhere to frameRate if set");
+
 [
  {video: true},
  {video: true, audio: false},