Import wpt/webmessaging tests

third_party/WebKit/LayoutTests/external/wpt/webmessaging/broadcastchannel/resources/worker.js

+var c;
+
+function handler(e, reply) {
+  if (e.data.ping) {
+    c.postMessage(e.data.ping);
+    return;
+  }
+  if (e.data.blob) {
+    c.postMessage({blob: new Blob(e.data.blob)});
+  }
+  c = new BroadcastChannel(e.data.channel);
+  let messages = [];
+  c.onmessage = e => {
+      messages.push(e.data);
+      if (e.data == 'done')
+        reply(messages);
+    };
+  c.postMessage('from worker');
+}
+
+onmessage = e => handler(e, postMessage);
+
+onconnect = e => {
+  let port = e.ports[0];
+  port.onmessage = e => handler(e, msg => port.postMessage(msg));
+};