[wrapper-tracing] For a worker, mark relative to its wrapper map.

third_party/WebKit/LayoutTests/fast/workers/resources/worker-xhr-onerror.js

+(function () {
+    var xhr = new XMLHttpRequest();
+    xhr.open('GET', 'does-not-exist.js');
+    xhr.onerror = () => { postMessage('done'); };
+    xhr.send();
+})();
+// |xhr| isn't in scope, but it will signal pending activity
+// when the GC runs, hence will be kept alive and allowed to
+// run to completion.
+self.gc();