Adds USB writing for OS X.

chrome/utility/image_writer/image_writer_stub.cc

 // 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.
 
 #include "chrome/utility/image_writer/image_writer.h"
 
 // This file contains the default version of the platform-specific methods of
 // the ImageWriter.  Add new platforms by creating a new version of these
 // methods and updating the compliation rules appropriately.
 namespace image_writer {
 
 bool ImageWriter::IsValidDevice() {
   NOTIMPLEMENTED();
   return false;
 }
 
-bool ImageWriter::UnmountVolumes() {
+void ImageWriter::UnmountVolumes(const base::Closure& continuation) {
   NOTIMPLEMENTED();
-  return false;
+  return;
+}
+
+bool ImageWriter::OpenDevice() {
+  device_file_.Initialize(
+      device_path_,
+      base::File::FLAG_OPEN | base::File::FLAG_READ | base::File::FLAG_WRITE |
+          base::File::FLAG_EXCLUSIVE_READ | base::File::FLAG_EXCLUSIVE_WRITE);
+  return device_file_.IsValid();
 }
 
 }  // namespace image_writer