Bug 1582509 - part2 : modify and enable wpt 'positionstate.html'. r=baku

According to [1], the playback rate can be either positive or negative, the only unsupported value is zero.

[1] https://w3c.github.io/mediasession/#dom-mediapositionstate-playbackrate

Differential Revision: https://phabricator.services.mozilla.com/D66501

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

testing/web-platform/meta/mediasession/idlharness.window.js.ini

@@ -1,10 +0,0 @@
-prefs: [dom.media.mediasession.enabled:true]
-[idlharness.window.html]
-  [MediaSession interface: navigator.mediaSession must inherit property "setPositionState(optional MediaPositionState)" with the proper type]
-    expected: FAIL
-
-  [MediaSession interface: operation setPositionState(optional MediaPositionState)]
-    expected: FAIL
-
-  [MediaSession interface: calling setPositionState(optional MediaPositionState) on navigator.mediaSession with too few arguments must throw TypeError]
-    expected: FAIL

testing/web-platform/meta/mediasession/positionstate.html.ini

@@ -1,19 +1,2 @@
+prefs: [dom.media.mediasession.enabled:true]
 [positionstate.html]
-  [Test setPositionState with a null value]
-    expected: FAIL
-
-  [Test setPositionState with a valid value for forward playback]
-    expected: FAIL
-
-  [Test setPositionState with optional position]
-    expected: FAIL
-
-  [Test setPositionState with optional playback rate]
-    expected: FAIL
-
-  [Test setPositionState with zero duration]
-    expected: FAIL
-
-  [Test setPositionState with only duration]
-    expected: FAIL
-

testing/web-platform/tests/mediasession/positionstate.html

@@ -42,16 +42,12 @@ test(() => {
 }, 'Test setPositionState with zero duration');
 
 test(() => {
-  assert_throws_js(
-      TypeError,
-      _ => {
-        window.navigator.mediaSession.setPositionState({
-          duration: 60.9,
-          position: 10.1,
-          playbackRate: -2.0
-        });
-      });
-}, 'Test setPositionState throws a TypeError if playback rate is negative');
+  window.navigator.mediaSession.setPositionState({
+    duration: 60.9,
+    position: 10.1,
+    playbackRate: -2.0
+  });
+}, 'Test setPositionState with negative playback rate');
 
 test(() => {
   assert_throws_js(