WebMessaging: Send transferable ArrayBuffers by copy-and-neuter semantics

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/postmessage-transferables-worker.js

+var messageHandler = function(port, e) {
+    var text_decoder = new TextDecoder;
+    port.postMessage({
+      content: text_decoder.decode(e.data),
+      byteLength: e.data.byteLength
+    });
+
+    // Send back the array buffer via Client.postMessage.
+    port.postMessage(e.data, [e.data.buffer]);
+
+    port.postMessage({
+      content: text_decoder.decode(e.data),
+      byteLength: e.data.byteLength
+    });
+};
+
+self.addEventListener('message', e => {
+    if (e.ports[0]) {
+      // Wait for messages sent via MessagePort.
+      e.ports[0].onmessage = messageHandler.bind(null, e.ports[0]);
+      return;
+    }
+    messageHandler(e.source, e);
+  });