[unified-heap] Fix media/encrypted-media/* liveness layout tests

Update asyncGC function to the latest version to guarantee that GCs are
indeed run without stack.

Bug: chromium:843903
Change-Id: Iaa61f27cacd99854665358720d2354522b756302
Reviewed-on: https://chromium-review.googlesource.com/1238623Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593550}

third_party/WebKit/LayoutTests/media/encrypted-media/encrypted-media-utils.js

@@ -118,10 +118,18 @@ function waitForEventAndRunStep(eventName, element, func, stepTest)
 
 // Copied from LayoutTests/resources/js-test.js.
 // See it for details of why this is necessary.
-function asyncGC(callback)
-{
-    GCController.collectAll();
-    setTimeout(callback, 0);
+function asyncGC(callback) {
+  if (!callback) {
+      return new Promise(resolve => asyncGC(resolve));
+  }
+  var documentsBefore = internals.numberOfLiveDocuments();
+  GCController.asyncCollectAll(function () {
+      var documentsAfter = internals.numberOfLiveDocuments();
+      if (documentsAfter < documentsBefore)
+          asyncGC(callback);
+      else
+          callback();
+  });
 }
 
 function createGCPromise()