Disallow certain blocking DOM calls during microtask execution.

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

+onmessage = function(evt) {
+    new Promise(function(res,rej) { res(); }).then(function() {
+        var xhr = new XMLHttpRequest();
+        xhr.open("GET", "http://example.test", false);
+        try {
+            xhr.send();
+            console.log("FAIL: xhr.send() didn't throw");
+        } catch (e) {
+            console.log("PASS: xhr.send() throws as expected");
+        }
+        postMessage("done");
+    });
+};