wpt: Fix script to call onAllClosed properly

test.step_func was only referencing onAllClosed, but never calling it.

Tested with the clearkey-mp4-playback-temporary-events.https.html test,
which was previously failing, but with this changed, it passed.

Bug: 349649084
Change-Id: I5230fa5d0ea66ee2bfdeeb05a653e375852c886c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6410055
Reviewed-by: Feras Aldahlawi <[email protected]>
Commit-Queue: Vikram Pasupathy <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1439819}

Author: gvking

encrypted-media/scripts/playback-temporary-events.js

@@ -111,7 +111,7 @@ function runTest(config,qualifier) {
                 closeMethodPromise.then(onClosed);
 
                 Promise.all([ closedAttributePromise, closeMethodPromise ]).then(function() {
-                    test.step_func(onAllClosed);
+                    test.step_func(onAllClosed)();
                 }).catch(onFailure);
             }
         }