Bug 1507675 - Drop the second argument for then(). r=boris

The second argument should be a callback for reject case.

Differential Revision: https://phabricator.services.mozilla.com/D12086

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

dom/animation/test/chrome/test_animation_performance_warning.html

@@ -332,7 +332,7 @@ function testSetOfGeometricProperties() {
               warning: 'CompositorAnimationWarningTransformWithSyncGeometricAnimations'
             }
           ]);
-      }, 'Transform animation should be run on the main thread');
+      });
     }, `${property} is treated as a geometric property`);
   });
 }