Fix a bug in data_sender.js.

extensions/renderer/resources/data_sender.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 define('data_sender', [
     'async_waiter',
     'device/serial/data_stream.mojom',
     'device/serial/data_stream_serialization.mojom',
     'mojo/public/js/bindings/core',
     'mojo/public/js/bindings/router',
@@ skipping 278 matching lines @@
    * resolves.
    * @return {!Promise.<SerializedDataSender>} A promise that will resolve to
    *     the serialization of this DataSender. If this DataSender has shut down,
    *     the promise will resolve to null.
    */
   DataSender.prototype.serialize = function() {
     if (this.shutDown_)
       return Promise.resolve(null);
 
     var readyToSerialize = Promise.resolve();
-    if (this.pendingSends_.length) {
+    if (this.pendingSends_.length || this.sendsAwaitingAck_.length) {
       if (this.pendingCancel_)
         readyToSerialize = this.cancelPromise_;
       else
         readyToSerialize = this.cancel(this.fatalErrorValue_);
     }
     return readyToSerialize.then(function() {
       this.waiter_.stop();
       var serialized = new serialization.SerializedDataSender();
       serialized.sink = this.router_.connector_.handle_,
       serialized.data_pipe = this.sendPipe_,
@@ skipping 158 matching lines @@
       // Note: Only the first PendingSend in |pendingSends_| will have data to
       // flush as only the first can have written data to the data pipe.
       bytesToFlush += result.bytesToFlush;
     }
     this.callCancelCallback_();
     return Promise.resolve({bytes_to_flush: bytesToFlush});
   };
 
   return {DataSender: DataSender};
 });