Adds USB writing for OS X.

chrome/utility/image_writer/image_writer_unittest.cc

Index: chrome/utility/image_writer/image_writer_unittest.cc
diff --git a/chrome/utility/image_writer/image_writer_unittest.cc b/chrome/utility/image_writer/image_writer_unittest.cc
index 0cc0c5bc9518119298e583b366aedad910a32312..0df210b7642d0cfb49a383ff1ec39daae34ce423 100644
--- a/chrome/utility/image_writer/image_writer_unittest.cc
+++ b/chrome/utility/image_writer/image_writer_unittest.cc
@@ -227,4 +227,18 @@ TEST_F(ImageWriterUtilityTest, WriteWithVerifySuccessful) {
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_F(ImageWriterUtilityTest, UnmountMac) {
+  base::RunLoop loop;
+  const base::Closure quit_closure = loop.QuitClosure();
+
+  VerifyingHandler mock_handler;
+  ImageWriter image_writer(&mock_handler, image_path_, base::FilePath("disk3"));
+
+  mock_handler.image_writer_ = &image_writer;
+
+  image_writer.UnmountVolumes(quit_closure);
+
+  loop.Run();
+}
+
 }  // namespace image_writer